Find and kill the Green Knight and kill him. His head will be left behind, get head.
In the room 'The Tower of Ynys yr Avalon' which is near Glastonbury (HUNT GLASTONBURY will get you close)
The path is the hopping through hedgerows rooms, leading north with a couple up directions. At the top is an altar.
PUT HEAD ALTAR
SAY I AM LOOKING FOR PASSAGE (you cannot be grouped for this)
This will get you into Avalon, which is needed to completed the quest for Excalibur and the holy grail.