The Ideal Masters are first introduced in Dawnguard as the 'proprietors' or 'landlords' of the Soul Cairn.

Though they make their first appearance in DG, I'd like to discuss them in more general terms.

To the best of my understanding, they've never been mentioned before in Lore, and thus are unknown in the older TES episodes. The fact that the dragon Durnehviir knows of them is interesting. Also, various necromancers have learned about them over time and had relationships with them. Finally, Molag Bal must know who they are, because he created vampires, and some of his original creations seem to have access to the Soul Cairn (as we learn in DG.)

However, their real identity is still pretty mysterious. The Soul Cairn is apparently part of Oblivion, but clearly quite seperate from the rest of it. Also, even though the Soul Cairn is in Oblivion, there is no mention (at least that I have read) of the Ideal Masters being Daedra.

They are clearly sinister - after all, they harvest souls relentlessly, and they make only the dirtiest, most deceptive deals with individuals in order to cheat and enslave them.

I think we need DLC that explores them more. I would love it if such a DLC also gave us a chance to destroy them - they have it coming.

Does anyone have any other thoughts on these Ideal Masters?

The Ideal Masters were mentioned in Battlespire, and Battlespire is the game in which we first got the visit the soul cairn.

but no, there really isnt any hard lore for what the ideal masters are, or what they really want.

INTERESTING! Could you expand on this?

How different was the soul cairn in Battlespire?

What role did the Ideal Masters play in Battlespire?

I never played Battlespire, so I dont really know, I only know they were in it.