Heya. I'm going to add my voice to the chorus of people saying the last line works. The whole prologue feels evocative of the Wizard of the Pigeons by Megan Lindholm, what with the fantasy world building with ethereal language and sentence structure followed by a sharp jolt to impress upon the reader that the word *urban* in urban-fantasy is the one to which one needs to pay attention.

I also think that the use of 'but' twice in the "But he wouldn't, (but he had once)" is a bit awkward. I get that the prologue is supposed to be in short snippets, giving you short glimpses into a wider world, but the use of two 'but's twice in quick succesion doesn't give me a sence of disconnect from the situation to enhance the otherworldy quality of it, it instead draws me out of the book altogether.

I personally think, and feel free to ignore me, that it would work better as either

"He could steal your soul if you let him draw you. He wouldn't, but he had once."

or

He could steal your soul if you let him draw you, but he wouldn't (though he had once).