Looks pretty good. I like how you worked in "they could satiate them on disintegrating rocks, but living matter is more nutritious, and intelligent living matter even more so."
A couple more refinements:
(1) To avoid confusion about the breath weapon, I think it may be worth copying some stuff from the Disintegrate spell:
"Any creature reduced to 0 or fewer hit points by this spell is entirely disintegrated, leaving behind only a trace of fine dust. A disintegrated creature’s equipment is unaffected.
When used against an object, the ray simply disintegrates nonliving matter up to a depth of 10 feet. Thus, the spell may disintegrate only part of any very large object or structure targeted. The breath weapon affects even objects constructed entirely of force, such as forceful hand or a wall of force, but not magical effects such as a globe of invulnerability or an antimagic field."
For esthetic purposes, we might as well follow the Disintegrate spell and scale the breath weapon in d6's, topping out at 40d6 (140 avg) instead of 24d10 (132 avg). Start at 7d6, add 3d6 per age category. (It's also more likely that DM's will have a bunch of d6's at hand than d10's.)
(2) We could have "Nullifying Breath" come into play only when the target would have other wise been totally disintegrated, i.e. reduced to 0 or fewer hit points. Makes perfect sense, and it's just a *little* less nasty...
(3) Semi-Astral: I would rephrase it as "While xxxs are living in the astral plane, their constant connection to the Material Plane allows them to..." (or maybe better "... subjects them to...")
(4) Auto-Quicken looks good, but I woulf prefer copying some of the Astral Plane language:
"Because the XXX exists on the Astral Plane, it may use all its spells and spell-like abilities as if they were improved by the Quicken Spell feat. Already quickened spells and spell-like abilities are unaffected, as are spells from magic items. Spells so quickened are still prepared and cast at their unmodified level. As with the Quicken Spell feat, only one quickened spell can be cast per round. A XXX is considered to have..."
(5) Multiplanar Rift: how exactly do you see this working? Could use some more explanation here. My interpretation was that it would shift its "rift" to the interface between the Astral and a different plane. It would still be a rift *to* the Astral, just *from* a plane other than the Material. Your wording would allow it to send creatures from, say, the Material to an Elemental plane. That means it's "skin" would be on the Material-Elemental interface, while its body is still on the Astral, which seems a little less consistent...
(6) I think we still need some additional benefits for the XXX on the Astral Plane itself. Right now it seems more efficient on the Material (better maneuverability, and can "suck" opponents towards itself). Why would it bother shifting opponents to the Astral? We also need to work out its non-breath-weapon attacks. Does it do disintegrating damage with it's claws and bite as well? I can't think of any incorporeal creatures that do regular physical hp damage...
Except that the disintegration has to kill the victim, not merely bring it down to 0 hp. (for some reason, I thought disintegrate would have to bring creatures down to -10, too, and the 0 would only apply to objects. Guess that is where part of my confusion comes from...)
