Maybe the soul cairn is a splinter of his realm, and the ideal masters are beings allowed to continue existing/ruling because they offer the collected souls to him. It was speculated that souls were treated as a type of currency there. Used for paying the landlord his rent perhaps? True enough. I'll have to remember that if it ever comes up again. I'm still more partial to the idea that a Vampire's soul is constantly being drained away, and that their 'feeding' is as much replenishing that power as the need for the actual blood. It would explain why they don't have to be partially soul-trapped to enter the Soul Cairn, since we can assume that as Undead they are already bound to it. Which makes one wonder what Molag Bal's link to the Soul Cairn, and the Ideal Masters, is. Wait, what's this about the Soul Cairn being related to Coldharbour? Molag Bal's the Daedra of Domination, I don't think he would be involved at all with the S.C. If anything, Sithis would be connected to the soul Cairn and the Ideal Masters, Seeing as how one of the original deities of Mundus, and the embodiment of nothingness/death.