Zardnaar
Legend
You're description of Rajaat matches a lot of warlock patrons honestly. It doesn't make a lot of sense in real terms for a god like Tharizdun to make a pact with someone, but that's mechanically how it can work. Same reason for why Rajaat would want to give someone a pact; vague backstory about how he sees this PC as the key to breaking free of his prison.
He gets killed in the Prism Pentad, but let's be real that's not going to be canon again.
The idea that there are no beings in Dark Sun powerful enough to support a warlock is frankly a little ridiculous.
It's not the power levels but PCs shouldn't have access to those patrons. At best the class should be NPCs only.
Sorcerer Kings as patrons is also silly since they should be defilers and having your Templars wandering around cities us also silly. Especially how the typical Athasian responds to defiling magic. No Sorcerer King is going to want a large amount of preservers serving them and they can only support a handful of pet defilers via trees of life.
It's also partly why they started using Templars in the first place. They weren't defilers.