I like it, sounds cool and origonal.
Stillit puts an interesting twist on this critter. I Still think 1 or 2 SLA might be in order. Teleport would be good eanough i think.
Sure those all sound good to me. What about some evil spells? Unholy Blight Protection with good. Just staples that most powerful demons have.
A unique obyrith (without demon lord status) is pretty good, though I believe those also turn you insane if you look at them. Is it reasonable to add that?
I will have to go through those SLA suggestions later.