HOW TO GET IT
It is a RANDOM drop from completing Shiro-4 bounties.
Step 1: Echoes of Darkness: Face "The Wretched Eye" and defeat Kovik, the Devil Splicers' high priest.
Step 2: Harvest of the Lesser: Wipe out Fallen in the Plaugelands to feed the weapon's appetite. (It was about 500).
Step 3: Feast Upon the Light: Use Void Damage to defeat Guardians in the Crucible. (125 kills or so) Deaths DO NOT set you back!!! PRIVATE MATCHES WILL NOT WORK
Step 4: Cleansed by The Light: Consume Motes of Light to Cleanse yourself and the weapon. (Only 5 motes)
Step 5: Where it Began: Find a Skeleton Key and open the Strike Hoard at the end of "The Abomination Heist" Strike. (You can get a free Skeleton Key from the Quest "None are Perfect")
Step 6: Receive Thorn