That person was incorrect. Every time you turn in a bounty of any kind (Crucible, Vanguard, even Queen's or Iron Banner as far as I can tell), there is a small chance the game will reward you with a Vanguard Mentor Missive alongside the normal reputation and EXP. There's no known way, as far as I know, to increase your odds of getting a specific bounty and it is random when you get it. Just turn in as many bounties as you can. I had none for a long time, then I got one turning in 3 Vanguard bounties and another turning in a Queen's Wrath bounty. While it would make sense that doing the summoning pits enough times would give you the Thorn bounty... you aren't even given a specific bounty. You're given a choice between multiple bounties. It's just a random reward for clearing any bounty. Once you [i]do[/i] get the bounty, you do have to clear the Summoning Pits as the 1st part of it, though. Also, there are two sets of Exotic bounties. It's possible even when you get a missive that Thorn (A LIGHT IN THE DARK) won't be a choice. If it isn't one, I reccommend "SHATTERED MEMORY FRAGMENT" instead, which gives you the Pocket Infinity, which is a Fusion Rifle that fires in full auto. Not BS full auto like most guns in this game; after the first charge time, it just fires burst after burst. Infinity requires you to dismantle 10 Rare Fusion Rifles at one point, though. Might want to buy those if you have some spare glimmer. I bought 10 and stuck them in the vault for when I get it. The Gunsmith randomly sells them.