The Ideal Masters are non-corporeal beings that rule over Soul Cairn. They deceive mortal necromancers and take their souls.
Powers and Stats
Name: The Ideal Masters
Origin: The Elder Scrolls
Classification: Rulers of the Soul Crain
Attack Potency: Unknown (According to Battlespire they entered Oblivion as immortals and had taken a realm of Oblivion then used it to create the Soul Cairn, however if their power is at all comparable to the Daedric Prince's is unknown, and whether this realm they speak of is the size of the Daedric Prince's realm is also unknown.)
Lifting Strength: Unknown, possibly None (As non-physical beings)
Striking Strength: Unknown, possibly None (As non-physical beings)
Durability: Unknown (Their nature of existence should make them hard to kill)
Stamina: Presumably limitless
Range: Multiversal (Can interact with Mundus)
Standard Equipment: Unkown
Intelligence: Extremely high (They have deceived countless mortals, were able to transcend their physical forms and become pure soul energy)
Weaknesses: They have an extreme hunger for souls, as such it can be used to a necromancer's advantage when bargaining with them. The degree of which they have influence over the TES Universe is unknown
Note: The actual power of the Ideal Masters are unknown. The only mentions and knowledge we have of them are mentions during the events of Dawnguard and some cutscenes during Battlespire. As such most information about their power is vague and "makes sense" speculation.
According to the Battlespire Athenaeum, the Ideal Masters exist as disembodied entities living in a distant platonic ideal. Supposedly they have long undergone rituals to create eternal persistent consciousness.