Details are: Xander is in Africa and is negotiating with a shaman/tribal elder for permission to take a slayer back home with him. To do so he must speak to the tribe's ancestoral spirits. He goes through a ritual where he is burned on his chest with a fire hot spear, Xander then speaks to the spirits. Unfortunately the spirits from his own past hijack the ritual and Xander ends up being burnt 5 or 6 times until the african spirits get their chance. Xander becomes a fully fledged shaman with many spirit guides, including Spike.
I hope this rings a bell with someone, I would love to read it again.