Ok, so this gamemode is kinda, well, on the odd side if you would say. Pretty much once you spawn the might poro king you get the king to tank your tower. The king is a giant poro that has like 5,000 HP. He looks like this:
Open this to see the fancy Poro King.

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The game mode is Blind Draft, on Howling Abyss (ARAM map for those familiar with that.)
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I have also noticed you have summoner icons also coming in with this release, I do not know how you unlock them as they are avaliable in the PBE Shop for 1 IP. But they change the appearance of poros that follow you in-game when you spawn.
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So, here are the objectives and cool features this gamemode will have, but instead of typing it all myself, riot has already made a topic on the PBE forums with the objectives of the game.

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Legend of the King Poro

This years Snowdown brings with it our next featured game mode and a legend. The Legend of the Poro King. Weve dramatically changed what fights on the Howling Abyss will look like, so lets get straight into the rules:

Played on the Howling Abyss, Legend of the Poro King is a 5v5 Blind Draft game mode, where your goal is still to destroy the enemy teams nexus. However, youll have some Poro companions with you (to throw at each other of course), be diving deep to make #bigplays and work together to summon some help from the legendary Poro King himself.

New summoner spells
When playing the Legend of the Poro King, your regular summoner spells are replaced with two new ones, made just for the mode:

1) Poro Toss / Poro Dash: (20sec cooldown)
Throw a Poro a long distance, dealing true damage to the first enemy unit hit. This ability can be recast for 3 seconds after it hits an enemy to cast Poro Dash, which will dash your champion to the target hit. Dashing to the target will reduce the cooldown of Poro Toss by 5 seconds.

2) To the King!: (10sec cooldown)
Passive: Hitting an enemy champion with a Poro gives your team a Poro Mark. Upon reaching 10 Poro Marks, your team summons the Poro King to fight alongside them. While the Poro King is active, no Poro Marks can be scored by either team.

Active: Quickly dash to the Poro Kings side. Can only be cast when the Poro King is summoned for your team.

The Poro King
The Poro King is benevolent and will aid whichever team has summoned him. He can be dispelled, but only to return again when enough favour has been curried with him. As a tanky siege minion, he brings an impressive power boost to whichever team hes aiding. His powers include:

-- Tanky siege minion with a LOT of health. Let him be your front line!
-- The Poro King absorbs all Poros within his range being thrown by the enemy team, making him a powerful pushing force.
-- The Poro King will heal nearby allied champions every 5 seconds for health and a smaller amount of mana based on the teams average level.
-- Every 5 seconds, the Poro King will lob out Poros in an arc around him, dealing magic damage to enemies where they land.