Plasm

Plasms come in earth, air, fire and water, each either human-sized or giant. All are skeletons with menacing, glowing yellow eyes. Fire plasms are charred bones laced with flickering flame; water plasms have bones of solid water that waver as they walk; air plasms are clear and seen only in motion; earth plasms look most like ordinary skeletons, hung with gobs of sticky black dirt. They can speak with any intelligent creature: earth rumbles, air whistles, fire crackles and hisses, water gurgles.

They attack on sight with two claws, 2d6 each for the smaller kind and 3d6 for giants. Poison and mundane weapons do nothing, and a magic weapon inflicts only its bonus, so a +5 holy avenger deals 5.

Anywhere but the Ethereal and its home plane, a plasm loses 1 Hit Die per round and vanishes when none remain. Feeding on its element regenerates 1 hp per round but takes its whole round. Spells of its own element feed it instead: for 3d6 rounds it gains 1 Hit Die per level of the caster (or of a device's casting level), adjusting its THAC0, saves and hit points to match. So a fire plasm struck by a fireball gains 5 HD when the caster is 5th level, or 6 HD when the fireball comes from a wand.

Once per turn it can sacrifice 10 hit points to wrap itself in a 30-foot-diameter elemental cloud for 1d6 rounds, dealing 20 points per round to all others inside (save vs. breath weapon for half). It usually does this while fleeing, but one fattened by enemy spells spends hit points freely.

Plasms are renegades of the Elemental Planes who serve no elemental lord; true elementals attack them on sight. Gates sometimes bring them to the Prime Material. They have no society and ally or betray at whim. New plasms appear when an ethereal storm drifts too near an elemental border, rips loose some matter and fuses it with ether.

They serve only as destroyers on the Prime and live on morsels of their own element.