You are to find and kill a traitor named Scar-Tail, who's hiding in Bogwater, far to the south. The Dark gift is available as a Dark Brotherhood quest reward. Pickup the first quest of the Dark Brotherhood in an outlaw refuge, go to Anvil and kill someone, complete the quest to get the Blade of Woe at the Lighthouse. I did get quite a few bounties on my head as well when I was a member of The Dark Brotherhood.
