This is a very old story, and I no longer have a link or any real info on it. Angel(us) and Xander are trapped in another dimension or realm. They end up working together, becoming lovers and/or mates, and rule the place for, I think, thousands of years. Long enough that they pretty much forget Sunnydale and the people there. Then, they are called back for some reason.
It could be related to when Acathla opened and Angel was sent to hell, but I can't be sure about that.
Any ideas? Thanks.
ETA: Just remembered a bit more. The people, or beings, that Angel and Xander ruled over didn't want them to leave. And I seem to recall X and A standing on a ledge, or balcony, looking over their kingdom and remembering their years there.