This is version one. The name is kind of a placeholder until I can think of something better. As you can see, he isn't actually any more complicated than a normal Warlock - he's actually a little simpler, because he doesn't have any of the weird movement-based abilities.
I may rebuild him as a human...while I really dig on the idea of playing a Tiefling, and I get some cool stuff out of it, the Human racial abilities would really shore up his weaknesses.
Background: Damian was a dashing and successful merchant. He led caravans of goods and ships with holds full to bursting across the uninhabited lands, with a smile and a silver tongue. He turned great profits and had a fine reputation. One day, though, an onrushing horde of Gnolls charged forth from the treeline. His guards were annihilated, and he was left for dead. He was one of the lucky ones.
While he lay there, his life flickering like a candle as the sun went down and the snow started to fall, a pale and dark-haired woman came to him. She said that she had many enemies. That the dark gods were jealous of her power and position. She offered to give Damian back his life, to do with as he wished. There was only one condition: He must send to her the souls of undead, and the disciples of her enemies among the gods. Like a river the souls must flow! Never-ending, a roaring flood of souls! He would reave the power of her enemies out from under them, as a scythe cuts wheat, as great nets pull forth fish from the ocean.
Damian has found the task infinitely more to his liking than he would have suspected.