All credit goes to the original founder on reddit. Link is at the bottom. You get the quest randomly when turning in bounties to Shiro at Felwinter's Peak. From that the steps are: Step 1: Echoes of Darkness: Face "The Wretched Eye" and defeat Kovik, the Devil Splicers' high priest. Doesn't matter what difficulty Step 2: Harvest of the Lesser: Wipe out Fallen in the Plaugelands to feed the weapon's appetite. Quite a bit of splicers have to be killed. Step 3: Feast Upon the Light: Use Void Damage to defeat Guardians in the Crucible. Private matches don't count. (No death penalty) Step 4: Cleansed by The Light: Consume Motes of Light to Cleanse yourself and the weapon. 5 motes. Step 5: Where it Began: Find a Skeleton Key and open the Strike Hoard at the end of "The Abomination Heist" Strike. And thats it! Now I just gotta get the quest to drop. https://m.reddit.com/r/DestinyTheGame/comments/53oc0r/can_confirm_how_to_get_year_3_thorn_spoilers/link