The Deep by John Crowley
For many generations the Just have been at war with the Protectors. In their strange world, supported by a huge pillar poised in the vast and mysterious Deep, ritual bloodshed and sorcery have obsessed the inhabitants since the beginning of time.
Half human, half machine, sexless and hairless, the Visitor from the skies enters the world on a mission unknown even to himself. Is he a peacemaker between the warrior clans, an observer, or, with his phenomenal qualities, a warrior himself, the likes of which this planet has never seen before?
Only time can tell, and time is something that his makers have not allowed for ...