So, does Theodora turn into daemon or...

[Бри.Уч.Кол.Мощ]

Idira just summoned one who pretends to be Theodora?
Considering Theodora got pretty heretical at the end of her life it's isn't a stretch she turned into daemon considering chaos can do that to mortal's soul.

[Pierogolepca]

No, it's just a Tzeentch daemon ♥♥♥♥♥♥♥ with Idira and the RT.

Theodora definitely dabbled but I don't think she was anywhere near being promoted to a daemon prince. It's the highest "honor" a mortal follower of Chaos can achieve.

[Garatgh Deloi]

That said, if she dabbled her soul could have been claimed by Tzeentch when she died. And according to some lore the souls claimed by the chaos gods become a part of that chaos god, and Tzeentch daemons are just a part of Tzeentch split off from the main entity.

So, following that logic, its not completely impossible that Theodora soul (or a part of it) is part of that daemon.

But i'd say its more likely just a Tzeentch daemon being a Tzeentch daemon (lying, fooling people, scheming, just being a asshat).