Interesting idea. You've clearly put a lot of thought into the backstory. I wonder if the CR might be a bit low though. Hold monster at will from a creature with perfect flight at a DC of 19 is fairly tough, I think. at level 9 the save itself is only at +6 for the best and +3 for the worst. (A ninth level fighter with no WIS bonus is looking to fail 75% of the time) Anyone without the ability to fly is basically just a sitting duck, waiting to fail that save. Then, boom execute.

additionally the execute ability, while cool, basically means instant death. A wizard, rolling D4 hit die and usually absent a decent con score, has a maximum of 36 hp at level 9, likely less then 30 to start. and of course, the execute allows the demon to sense if the character has less. That means, round one, from the air, mass hold person (DC 23, meaning a good save needs a 17 before wisdom), then swift action executes wizard. hell, if the party isn't already flying, it doesn't even need to open with mass hold person. It will do an average of 27 damage, or 54, or 81, requiring the level 9 wizard to make three DC 37 FORT saves or die. If it does decide to use hold person, or mass hold person, the next round, any target effected by the ability gets the same treatment. (hold person does leave you helpless right?)

Not saying any of this is bad. Its a mighty monster you've created here, and that was your goal. It just seems something this deadly needs a higher CR than 9. (Not that I'm an expert or anything just something to consider.)