Crazy Immortal Buffy

Hi,
I'm looking for a fic where Buffy becomes immortal after the spell that Willow did to resurrect her. I remember something about when the spell's done a certain way, meaning the smashing of the urn thing, it can make people immortal, but it's really hard to do. And in it Buffy is acting kinda crazy, and not the good kind , but going through like surges of slayer power and emotion and stuff. And I think at one point she literally rips a demon to pieces with her bare hands.
Thanks

That sounds like Thrall by Chase

That's great thanks. that's exactly the one I was looking for. It's good to get the chance to read it again. Thanks.