Introduction
This Guide is a WORK IN PROGRESS. I was inspired by Scarlet Lady to make a 100% completion guide for Undead Nightmare. This whole guide is based on the template used for the orginal 100% CG, give thanks to SL for the amazing work!
This is a collaborate effort, please thank everyone who helped, their hard work helped make this world a better place haha ;)

I'd like to thanks everyone in advance for their help. Not to mention all those who helped on the original!

100% Game Completion Check List
Need to do the following for 100% completion status
-Complete all 8 Main Story Missions
-Complete all 5 Side (Survivor) Missions
-Collect 2 Outfits (More than 2 in the game, but a specific 2 are needed for 100%)
-Get Legendary Status in/Complete all 4 Challenges (Hunting, Treasure, Sharpshooting, Horses of the Apocalypse).
-Locations Saved, you only have to save each location once - they don't all have to be saved at the same time.
-All 5 Graveyards Cleansed (All graveyards will be cleansed during story missions, so there's no need to go out of your way looking for them)
-Save all Missing Souls (10 missing souls in Fort Mercer and 6 in El Presidio)

Commonly Missed Things
Please PM/Contact me anything that you missed, and found out later, it could help someone out greatly!

Unicorn
The unicorn must be caught and broken at least once to achieve 100%
The unicorn can be found in the area north of Torquemada where you see the three horses on the map, which is also called Diez Coronas. (Same area as the Chupacabra)
Note* The only way to find the Unicorn is to complete the Undead Hunter Challenges & Four Horses of the Apocalypse.
You will not get any message telling you that you have found the unicorn nor will a blue circle on the map appear.
If your unicorn were to die you can still catch and break another in the same location.

Main Missions Walkthrough - Complete (MAJOR SPOILS)
Intro
1.After the sequence, kill Uncle by shooting him in the head.
2.Now, lasso and hogtie Abigail and Jack.
3.Then, ride towards the U marked on your map. After the MacDougal gets bitten, shoot both zombies (the Nastas and the professor) in the head.
4.Afterwards, quickly walk to the new U location on your map, without wasting bullets on zombies. Simply, stay out of their way.

BlackWater Graveyard
1.Ride towards the graveyard (marked on your map) and burn the marked coffin. After the short sequence, burn the remaining coffins marked on your map.
2.Now kill as many zombies as possible on the graveyard.
3.When the next sequence is over, kill all the zombies marked on your map.

Town Safety: Blackwater
1.Ride towards the U on your map for the next mission.
2.You need to help out people in the town. Run towards the marked location and climb up the ladder.
3.Now, you need to get sniper rifle ammo. Get back down and move towards the marked location. You have to enter the building. When you enter, go left, run up the stairs and towards the end of the room. There is a box on the floor with sniper rifle ammo.
4.Once you have the ammo, get back to the two men on the roof, give each of them ammo and shoot the remaining zombies.

Curious Tales from Blackwater
1.Walk towards the U on your map.
2.After the sequence search the green area for more survivors.
3.Your map will display two Us. Ride to the closer one to start your next mission.

Get back in that hole, partner
1.After talking to your old friend, ride towards the closest skull marked on your map.
2.As in the earlier mission, burn the marked coffins and then kill all remaining zombies on the graveyard.
3.Then, ride to the next closest scull marked on your map. And clear out the next graveyard.
4.Now, ride to the last scull marked on your map and clear the graveyard. Afterwards, return to Seth (the S on your map).

A Cure for Most of What Ails You
1.Ride to the other U on your map.
2.You now need to collect plants for West Dickens: 5 Desert Sage and 5 Violet Snowdrop.
3.Desert Sage can be found at Gaptooth Ridge. Mark that location with a waypoint and ride there. Now, slowly ride around the area to find the flowers. Flowers will appear as little white marks on your map once you are near them. When you see one, walk up to it and collect it.
4.Afterwards, set a waypoint to the northern part of Tall Trees. Ride there and collect 5 Violet Snowdrops.
5.Return to West Dickens to deliver the plants.
6.Then, ride to Rileys Charge (marked on your map) to pick up the old parts he was talking about.
7.Afterwards, deliver the parts to West Dickens (the W on the map).

American Imperialism
Ride to the W on your map.
1.After talking to West Dickens, ride to the marked location to obtain a uniform.
2.Help out the people at the marked location by killing all zombies. Then, get the uniform.
3.Now, ride to the train station. Kill all zombies at the station and get on the train.
4.During the train ride do not worry about the zombies on the side of the train.
5.Once the train stops, kill all zombies in front of you. Then, get back on the train and release its breaks.

Mother Superior Blues
1.Ride to the U in Mexico.
2.After you are done talking to the nun, walk into the town behind you and safe it. Talk to the leader, bring ammo to the people marked on your map and shoot the remaining zombies.
3.Afterwards, return to Mother Superior.
4.Now, go find an undead, hogtie it and bring it to Mother Superior.
5.Then, ride to the graveyard (the skull on your map), burn the coffins and kill all remaining zombies. You will have to kill quite a few to clean that graveyard. Keep running around and shoot as many as you can.

Mother Superior Blues (continued)
1.Ride to the M in Mexico and talk to Mother Superior in the church.

A Civilized Man
1.Ride to the R in Mexico.
2.Then, kill Reyes by shooting him in the head.
3.Now, walk towards the yellow mark on your map and enter the catacombs. Follow the path and take out all zombies along the way.
4.When you reach the very last room, kill all zombies within it. Afterwards, place the mask on the marked location.
5.Congratulations! you have completed the main missions of Red Dead Redemption Undead Nightmare! You may continue saving towns, finding missing persons, completing side missions and challenges.

Stranger Missions Walkthrough - Complete(MAJOR SPOILS)
Paternal Pride
1.You need to save MacFarlanes Ranch, before you can accept this mission.
2.Now walk to the green ? at MacFarlanes Ranch to accept the mission.
3.Walk towards the barn. In order to get in you need to climb up the ladder on the side of the barn and then jump onto the roof. You will find an entrance at the back of the roof.
4.Now kill all zombies in the barn as well as Bonnies father.
5.Then, return to Bonnie (the B on the map).

Dinner for Two
1.Save Amadillo and walk to the M on your map.
2.Now, search the green area for the two deputies and kill them.
3.Afterwards, return to the Marshal Johnson.

Birth of the Conservation Movement
1.You can find this mission at the green ? in Tall Trees. (Tanners Reach)
2.Ride around Tall Trees and kill six sasquatches. You will find them in the green areas marked on your map. I recommend using cation since bears and cougars exist in the area.
3.Choose to kill the last sasquatch or leave it alone and let it live

Missing Souls
*Note this is only the beginning mission needed to start off the missing persons/souls
1.Take the missing person poster at Ford Mercer.
2.Then, ride to the marked location and free the girl from the zombies.
3.Afterwards, ride back to Ford Mercer with her.
4.Continue saving missing persons at Fort Mercer and later on El Presidio (Seeing missing persons/souls)

Filth and Other Entertainment
1.Before being able to accept this mission, you have to safe the town near Plainview.
2.Now go to the green ?.
3.Then, ride to Gaptooth Ridge and find a zombie of the kind that spits a green substance. (I found mine in Agave Viejo as I already saved Gaptooth Breach)
4.Hogtie the zombie and put in on your horse. Then, ride back to the movie director.

Biographies and Lies
1.Ride to the green ? in Mexico.
2.You now have to get dynamite and bait. In order to do so, save Agave Viejo and Gaptooth Breach for dynamite. While bait is acquired from the mission "A cure for Most of What Ails You" and can be built after using up one Prairie Poppy and one Wild Feverfew to create a single bottle of Undead Bait.
3.Afterwards return to Casa Madrugada.

Hunting Challenges - Complete
To keep track of the number of Undead you've put to rest, press Triangle/Y on the challenge in your Journal. See Sharpshooter for some suggested locations.

Rank 1 - Kill 100 fresh undead & 25 Bolters.

Fresh undead are any who have been recently converted. Just think of these as "regular" undead - anyone who doesn't fall into one of the other three types is considered fresh. If you see an undead person running on all fours to try and get to you, that's a bolter. Nothing that can't be accomplished through regular story play.

Rank 2 - Kill 25 more Bruisers & 15 more Retchers.

Bruisers are the big, lumbering undead that sometimes charge at you and knock you down. Retchers have a greenish glow to them and spit acid at you. Another easy challenge.

Rank 3 - Use a single explosion of a Retcher to kill 2 other undead.

When a Retcher is killed, they will explode. For this rank, you need to make sure there are two other undead close enough to one when it explodes that they are killed by the explosion. An easy way to do this is find a Retcher in any of the suggested locations and throw down some bait. Immediately shoot the Retcher in the head when other undead are around it.

Rank 4 - Kill 2 undead bears, 5 undead wolves and 3 undead cougars using a torch.

Wolves: pretty much all over the place as you ride around the frontier, listen for the barking. Just keep moving around as you're waving the torch so you don't get knocked over, and you should be able to take a few of them down.
Cougars: can be found in multiple locations, but the best one I've read about is the area due north of Fort Mercer. (I myself found them at Bearclaw Camp, lol)
Bears: typically only found in Tall Trees, north of Manzanita Post.

If your having dieing troubles when facing cougars and bears simply ride around on Pestilence (if you don't have him any more, see 4HOTA Rank 5 challenge) waving the torch when you find a one, and you'll take them down fairly easily. Watch out for the undead who will simply pull you off your horse.

Rank 5 - Find and kill a Chupacabra - a sort of undead hyena that is very fast.

The Chupacabra is a mythical creature that can frequently be found in the area north of Torquemada where you see the three horses on the map (also called Diez Coronas), although it can spawn just about anywhere in Mexico, with other common locations being the areas around Chuparosa and Sepulcro. (He can't be found before Rank 5)

Sharpshooter Challenges - Complete
Most of these challenges require a large amount of undead. Prime locations are all towns (if still filled with zombies), Jorges Gap and both of the forts (Fort Mercer and El Presidio). If your having trouble grouping them use bait or just run in front of them so they start chasing you.

Rank 1
Objective: Kill 5 undead in 10 seconds using a gun.
Extremely easy to do at really any location. With Casual and Normal Aiming its a breeze but if your using Expert don't forget about Dead-Eye
Reward: Dead eye item

Rank 2
Objective: Headshot 10 undead in a row
No time limit on this one, but all headshots must be made in a row. Still on CA and NA its easy, so for Expert aim get a good angle, line up the zombies, and use dead eye with a weapon that has more than 10 rounds
Reward: Ammo and Dead eye item

Rank 3
Objective: Kill 5 flaming undead in 1 dead eye meter.
To light the zombies on fire run them over with War (see 4HOTA) or throw fire bottles at them (see Misc, weapons). Now all you have to do is line up the shots in DE
Another alternative to this is using phosphorus bullets which count as igniting the zombies.
Reward: Ammo

Rank 4
Objective: Kill one of each of the four undead types in one Dead Eye meter
Of the above locations Fort Mercer and El Presidio are your best bets to get this done. Stand on one of the walls overlooking the zombies and target one of each types in dead eye.
If your having trouble confusing the normal zombies with the crawlers look for design differences between the two. Crawlers will most likely have an arrow or an axe stuck in them, plus they have a more noticeable limp while walking.
Reward: Holy Water (Ammo if Holy Water is not unlocked)

Rank 5
Objective: Get 5 undead sniper kills in 8 seconds (can't use Dead-Eye).
This may seem hard but is extremely easy. You see up close executions count and so are a breeze to pull off. Go to a suggested location like Jorges Gap, (bait isn't necessary but use it if your having trouble) run up to a zombie and just press the trigger button without aiming. An animation where Marston puts the gun under their head and fires should happen. Quickly move on to the next zombie and the next. (I myself just ran up to some outside El Presidio and used the above strategy)
Reward: Carcano Rifle & Mauser Pistol

Treasure Challenge Locations - Complete
All treasures are under a pile of small rocks.

Treasure 1
Located north of MacFarlane's Ranch and west of Tall Trees

Treasure 2
Located east of Venter's Place and south of Coot's Chapel

Treasure 3
West of Scratching Post and south of Gaptooth Breach

Treasure 4
South of Sepulcro

Treasure 5
West of Casa Madrugada and between Agave Viejo

A video for those that can't find some
[ Register or Signin to view external links. ]

Horses of the Apocalypse - Complete
Each of the four horses is considered a mythical creature, so a blue circle will appear on your map along with a message telling you that a mythical creature has appeared any time you are near one. You break them just like you break any horse in the game. (Use lasso and stay on top of horse)

War - a fiery red horse whose hooves are perpetually engulfed in flame.

Located at Tumbleweed, Benedict Point, and Gaptooth Ridge in New Austin. He has also been found east and west of Armadillo in Cholla Springs, north of the MacFarlane's Ranch in Cholla Springs, and around Fort Mercer in Rio Bravo. (Sets zombies on fire)

Pestilence - a pale white horse covered in wounds and shrouded in a green disease cloud.

Located between Manzanita Post and Aurora Basin in Tall Trees. He has also been spotted south of Broken Tree in the Great Plains region, and west of Thieves' Landing in Hennigan's Stead. (Practically invincible)

Famine - a jet black horse that is covered in a swarm of locusts.

Located in the stretch of land between Chuparosa in Perdido and Tesoro Azul in Punta Orgullo. Famine can also be seen near Las Hermanas in Perdido, and outside Torquemada in Diez Coronas. (Practically unlimited stamina)

Death - a dark brown horse that leaves wispy entrails of ghostly fog in its wake.

It is awarded to you upon the completion of the Undead Nightmare campaign.
However he can appear after the other HOTA have been tamed.
There isn't a particularly frequent location he spawns at; it is pretty random, but searching the areas the other three Horses of the Apocalypse were found in is your best bet. (Kills zombies on touch)

Four Horses of the Apocalypse Rank 5

You are required to kill one of eachtype of zombie using Death's special skill. Just hop on Death and ride into the undead, as the slightest touch will cause their heads to instantly explode. You are rewarded the 'Blood pacts' of the other three Horses which allows you to whistle for them whenever you want.

Missing Souls/Persons - Complete
All persons must be saved for 100% (10 missing souls in Fort Mercer and 6 in El Presidio)

Names of persons--location--where to get information

Millicent Waterbury--Pleasance House--Fort Mercer
Mintie Cummings--Riley's Charge--Fort Mercer
Winona Picket--Lake Don Julio--Fort Mercer
Grover Boone--Venter's Place--Fort Mercer
Silas Gaskell--Critchley's Ranch--Fort Mercer
Willis Lassiter--Benedict Point--Fort Mercer
Chu Fook--Gaptooth Breach--Fort Mercer
Orison Pratt--Ridgewood Farm--Fort Mercer
Willie Oats--Plainview--Fort Mercer
Adrienne Lachance--Armadillo--Fort Mercer

Porfirio Guetierrez--Roca de Madera--El Presidio
Ramon Alvares--Torquemada--El Presidio
Lydia Morales--Las Hermanas--El Presidio
Rafael Carillo--Agave Viejo--El Presidio
Santiago Valenzuela--El Matadero--El Presidio
Merche Coronado--Casa Marugada--El Presidio

Outfits - Complete
The * marks the outfit needed for 100%

Union Suit (disappears after completion of UN story)
Prerequisite: Finish the first mission "Love in Time of Plague"
Benefit: None
Scraps: None

Cowboy Outfit
Prerequisite: Start the Game ( Default Outfit)
Benefit: None
Scraps: None

Rancher
Prerequisite: Start the Game
Benefit: None
Scraps: None

Poncho
Prerequisite: Start the Game
Benefit: None
Scraps: None

Duster Coat
Prerequisite: Start the Game
Benefit: None
Scraps: None

U.S Army (Required through Story)
Prerequisite: Finish the last segment of Nigel West Dicken's mission "A Cure for Most of what Ails You"
Benefit: None
Scraps: None

Undead Cowboy (replaces Cowboy outfit)
Prerequisite: Finish the UN Story
Benefit: None
Scraps: None

Undead Hunter (switches to Army of the Undead once you complete Story)
Prerequisite: to unlock information, finish one of the scraps
Benefit: a holy relic is placed in the player's inventory. When the relic is used, ammunition is replenished and consumable items are added to his inventory. The only drawback is that the relic can only be used once a day.
Scraps:
Scrap 1: Complete the Survivor Mission "Birth of the Conservation Movement".
Scrap 2: Complete the Survivor Mission "Filth and Light Entertainment".
Scrap 3: Complete the Survivor Mission "Missing Souls".
Scrap 4: Return one Missing Person to El Presidio.
Scrap 5: Find and break the mythical unicorn

Legend of the Apocalypse (switches to Legend of the Undead name once you complete story)
Prerequisite: Get legendary status for ALL Challenges.
Benefit: Increases the duration of phosphorus bullets.

Weapon Locations

You get all weapons as rewards throughout the game, so no need try to find them if your going to 100% (Except Semi Auto Shotgun)

Pistols

Cattleman: Given at the start of the game
Double-Action Revolver: Save the Tesero Azul Hideout
High Power Pistol: Complete all the tasks for the Doctor (random encounter)
Lemat Revolver: Complete Undead Hunter Rank 5
Mauser Pistol: Complete Undead Sharpshooter Rank 5
Schofield Revolver: Save Chuparosa
Semi-automatic Pistol: Return the first missing person back to Fort Mercer
Volcanic Pistol: Save Twin Rocks Hideout

Projectiles

Dynamite: Save either Gaptooth Breach Hideout or Agave Viejo
Fire Bottle: Save Thieves Landing
Holy Water: Obtained from Mother Superior in your second mission in Mexico. It can be built after
Tomahawk (L&K DLC required): Save Manzanita Post
Undead Bait: Acquired during the Nigel West Dickens mission "A Cure for Most of What Ails You." It can be built after
Boom Bait: Acquired from Landon Ricketts during the "Biographies and Lies" survivor mission. It can be built after

Rifles

Repeater Carbine: Obtained after cleansing the BlackWater graveyard for the mission "Curious Tales from BlackWater, USA"
Buffalo Rifle: Save Escalera
Evans Repeater: Complete Undead Hunter Rank 5
Henry Repeater: Save Nosalida
Bolt Action Rifle: Save Cochinay
Springfield Rifle: Save Ridgewood Farm
Winchester Repeater: Save MacFarlane's Ranch
Explosive Rifle (L&C DLC required): Save Rathskellar Fork

Shotguns

Double-barreled Shotgun: Given at the start of the game
Pump-action Shotgun: Save Torquemada
Sawed-off Shotgun: Given after completing "Dinner for two" survivor mission
Semi-auto Shotgun: Found in the Story's final mission at a short dead end hallway before the last set of stair (Miss-able weapon)


Sniper Rifles

Rolling Block Rifle: Save pacific Union Railroad Camp
Carcano Rifle: Complete Undead Sharpshooter Rank 5

The Downward Spiral 10G
Complete Curious Tales from Blackwater, USA Survivor Mission. (This is a story related trophy and cannot be missed)

Judge A Man By The 30G
Complete Cure For Most of What Ails You and Get Back in that Hole, Partner Survivor Missions. (This is a story related trophy and cannot be missed)

The Superior Dance 10G
Complete Mother Superior Blues Survivor Mission. (This is a story related trophy and cannot be missed)

Alls Right With the World 25G
Complete On a Pale Horse. (This is a story related trophy and cannot be missed)

Mad Martson: The Trail Warrior 10G
Attain Rank 5 in all Undead Nightmare Challenges.

Chupathingy 10G
Find and kill a chupacabra.

Fan Service 10G
Find and break a unicorn.

Six Years In the Making 10G
Find and kill a sasquatch.

Spinning Plates 25G
Have every territory saved at the same time during the Undead Nightmare. (A saved location is marked with a blue cross)
Required locations:

West Elizabeth (4)

-Blackwater-
-Manzanita Post-
-Cochinay-
-Pacific Union R.R. Camp-

New Austin (9)

-Thieves' Landing-
-MacFarlane's Ranch-
-Armadillo-
-Ridgewood Farm-
-Twin Rocks-
-Rathskeller Fork-
-Gaptooth Breach-
-Plainview-
-Fort Mercer-

Mexico (10)

-Escalera-
-Nosalida-
-Tesoro Azul-
-Las Hermanas-
-Chuparosa-
-Agave Viejo-
-Casa Madrugada-
-El Matadero-
-El Presidio-
-Torquemada-

Smoke That Skinwagon 10G
- Make it to wave 15 in Undead Overrun Gametype in Multiplayer.

Kingpin 10G
Fight off 8 unique players during your time on top during Land Grab in Multiplayer Free Roam.

Zeds Dead, Baby 40G
Attain 100% Game Completion statistic in Undead Nightmare.

Thanks to Scarlet Lady for everything
Thanks to DAVEYHASSELHOFF of ps3trophies.com (Both Guides)
Thanks to DiamondBackDylan for the zombie visuals
Thanks to Twyzted7 (Original Guide)
Thanks to Codenman (Original Guide)
Thanks to .Cody (Original Guide)
Thanks to JADO (Original Guide)
Thanks to MutantSquid (Original Guide)
Thanks to RedDeadFan (Original Guide)
Thanks to the FallenSaint (Original Guide)
Thanks to joeman1000 of GameSpot Forums (Original Guide)
Thanks to XxSKELETOxX for grammatical help. (Original Guide)
Thanks to Seth Briars and Whizz343 for trophy/achievement information (UN Guide)
Thanks to Xbox 360 achievements.org [ Register or Signin to view external links. ]
Thanks to -RaaawR- from TTG for being awesome
Thanks to Red Dead Wiki
Thanks to Mahalo.com
Thanks to Apache Kid