He is CR 25, because one of his legendary actions is to summom an ice devil, that can in turn summon other devils. He can literally sit back and create an army.Decent. I asked one of the people doing the previews how he differed form a normal pit fiend. And his CR was brought up.
I was impressed I thought he would be CR 21 or 22. But he is still on the Level of an Archduke.