Cleon
Legend
Yeah, that's kind of my point. I would think Ythog-Nthlei should have great immunities but more on par with the "standard" avatar demon lords/archdevils as opposed to the divine versions. After all, he's on the Prime Material and has been sleeping for eons. It seems he'd be a bit weak. Let's give him the "Beyond" abilities as well as immunity to energy drain. And that should probably be it.
You're forgetting that an ancient council of mages spent hundreds of years trying to destroy him. That suggest that at the very least he's immune to transformation, ability drain attacks, energy drain and other permanently incapacitating effects.
Hmm, I suppose it could be that permanently incapacitating effects like petrification, being ability drained to Wisdom 0, permanent paralysis/domination/psychic churergy and so forth just trigger his Beyond Life ability, so he "pops back" after a while. If that's the case, we just need to give the Beyond Life ability a tweak as well as giving him immunity to energy drain and ability drain.