I like this one a lot, and wish once more I ran a higher level game.
I have gotten to run one game with an undead black dragon that, when bloodied, decohered into:
1) a great big animate hurricane of jagged scales, lashing at adventurers with barbed whips of dragonskin;
2) a Phalagar-derived, spiderlike entity, dripping with acid and composed of the dragon's flesh and muscles and organs;
3) a difficult-terrain skeleton, conveniently right near the map's main chokepoint; and
4) a huge fractured draconic Vortex Wraith, attacking the party with spectral claws and trying to suck people off the great bridge with that vortex aura it has.