Thorn is an Exotic Hand Cannon weapon found in Destiny through the exotic weapon bounty mission line. If you to get a hand on it, you need to complete the Bounty Mission, which Im not sure to how to get, but if you do your bounty mission daily, you will get it soon.

In this mission you will have three options, the first is called A Light In The Dark, which is for the Exotic Hand Cannon, in the middle it called Toland Legacy, its for the Exotic Rifle and on the last called A Dubious Task, which is for the Exotic Shotgun.

Thorn Exotic Hand Cannon Weapon Guide

First of all you need to go to the Summoning Pits, after clearing the first boss you will unlock the chain to kill the 500 enemies.

The easy way to get it is, you need to go to the story mission The Dark Beyond at this mission you can easily kill the 500 enemies.

Here while killing the enemies, you will see a knight with the yellow heath bar DO NOT KILL IT just kill the red health bar enemies and let the knight kill you, you will re-spawn, so will all the enemies, so kill them again. It will take 15-20 mins to complete the task.

But remember one thing, if you kill the Knight the other enemies will not re-spawn.

The next part of the chain is to get 500 Void Points for that you need a weapon with the void damage, if you are defender titan of void type warlock you will have these kind of weapons. Everytime you will kill anyone with void damage weapon you will get the 5 points and if you die you will lose 2 points.

After getting the point go to The Speaker, bring a Mote of Light with you, because to continue you need to buy Infusion of Light which will cast a Mote of Light. After using the Infusion of Light, go and visit the warlock vanguard and pick up the final part of the bounty mission chain.

Now you need to go the special version of the summoning pits, which level is 26 and its very very hard.

Now you need to clear the pit and reach the boss, which is chained in the middle of the area, dont kill the chained boss, you need to keep him alive in order to spawn the required boss, which will take up to 10 minutes. After spawning the required boss, just kill him and you are done. You can just leave the chained boss and leave the pit.

Now head back to The Speaker and get your reward as Thorn Exotic Hand Cannon.