Thorn Year 3
Thorn returns in Destiny: Rise of Iron, so everyone who loved it (like me) and everyone who loathed it will be equally excited probably. Reddit user SteviaSteve was the first to document his steps to getting Thorn Year 3, but as of right now it seems to be a completely random quest that you receive.
On the topic of getting the mission, it seems to be coming your way from completing weekly missions and bounties.
Check out the Reddit post from SteviaSteve for a step by step guide of the mission.