Can't become Saoshyant because the Immortals have holdings in my Empire?

[Caspian Mortis]

I'm playing as a restored Persian Empire and I have already restored the High Priesthood, but there is a problem.
To become the Saoshyant you have to "completely control" the Persian Empire, which I do. However, there is this Holy Order of Immortals that is apparently not my vassal.

How do I make them my vassal so I can become Saoshyant? Anyone who knows this?

 

[Caspoi]

I'm playing as a restored Persian Empire and I have already restored the High Priesthood, but there is a problem.
To become the Saoshyant you have to "completely control" the Persian Empire, which I do. However, there is this Holy Order of Immortals that is apparently not my vassal.

How do I make them my vassal so I can become Saoshyant? Anyone who knows this?

Ask them, if you are emperor of Persia they should accept. A more extreme solution is to confiscate all of their lands but that costs a lot of prestige.

EDIT: I meant piety

 

[Jamoliso]

What Caspoi said. simply offer them vassalization. As an emperor they will accept for sure.

 

[Rationalsanity]

Yeah, unless the Grand Master got a kingdom or he personally hates you, he'll kneel if you ask.