Find and kill the Green Knight and kill him. His head will be left behind, get head.
Near the ArchBishop in Camelot where you are knighted for the realm is a path leading north with a couple up directions. At the top is an altar.
Put head altar
Say I am looking for passage
This will get you into Avalon, which is needed to completed the quest for Excalibur and the holy grail.