And why does he have valuable tomes that my Sylesties need to help him move? Anybody know? Anyone ever seen this guy?

He shows up in the main story in Umbra Forest. He's an elf with a lot of knowledge who has been a hermit for a while or something. It's been over a year since I played that section of the game so I don't remember exactly, but that's the gist of it?

Casius was an oracle that was banned by the fairies from the Sanctuary of Sarielle after he foresaw the fall of the sacred tree (I think that's what it was called?) The queen believed him and tried to protect him, but the rest of the fairies believed it to be blasphemy and threw him out anyways. He resides in the Enigmatic Grotto as a hermit, but helps you in the story after you take out a troll and get his staff back for him.