What you need to do is turn in 5 bounties at the same time. Its the most likely chance you will get one. After turning in multiple bounties (the quantity is just an "X" or variable) you get the possibility to obtain the selection of 3 random bounties. The reason i say 5 is because it gives you more possibilities to earn one. (5x"X"=4 possibilities) The bounty for Thorn is the one that will start you off saying something about a curse and will lead you to the summoning pits strike at any difficulty.