Yesterday I asked for feedback on partially immune creatures and Entombment.
Based on the comments, I think I'll go with partial immunity - the spell can have a partial effect, even if the victim is immune to the other part of the effect. In other words, spell effects are separable.
Now, none of this says a Black Reaver is even vulnerable to the spell at all. But there is at least one critter out there that can't be held, slept, petrified, or put into suspended animation. This may be the one - or not - but now I have a plan for when someone rolled a successful Entombment spell on him.
Thanks for the comments everyone.