So I said this once but DW ate my comment.

He could totally work in Sable's verse. He could be a stabilizing element towards sem and could totally be a part of the epic family that the Curator is in. Could even be tied into the Curator herself and there's your family death via potential supernatural creature. I'm thinking that the family/group/whatnot has the power negating ability and that's why they can work with the artifacts and whatnot and why Sable didn't take form until the curator died. Se is stuck in one form when near David and yeah. Stuff