Xaver

This intelligent creature is indistinguishable from a silver sword: a smooth metallic body, bulbous at the base, flat-sided and keen-edged along its length, tapering to a tip. Six faceted green eyes ring the base like hilt gems, and between each pair is an 8-inch wormlike leg it scuttles on and retracts instantly at any threat.

It has human-normal sight and 90-foot infravision but no hearing or smell, and makes no sound or heat. It absorbs sunlight and gases through its skin, lies still when anyone approaches, cannot corrode while alive, and is immune to rust monsters.

Xavers fight by spinning and slashing in a wild whirling dance, cutting with their razor edges. Metal passes straight through them as if they were not there. Heat and electricity do no damage to a xaver but flow through it at full effect into anyone and anything touching it. Poison and venom are useless, while cold deals an extra point per die.

Any iron-based metal that contacts a xaver cracks into shards one segment later, which the creature then tries to eat. It cannot suppress this power but can avoid touching metal if it chooses.

Xavers eat iron and alloys containing it, steel and adamantine included. Unlike rust monsters they are too clever to settle for a handful of tossed spikes when real armor and weapons are nearby. Big meals make them grow: a feast of multiple armor suits or a dozen-plus weapons may add an inch.

They can go years without food and live for centuries. Most are solitary, though wandering young, mated pairs or a few that have exhausted their food may form small family groups, and they never fight each other. Rocky lairs are typical, but some hide in treasure piles; a hoard that is initially found to hold armor, shields or weapons has a 4% chance of harboring a xaver still working through the metal.

Each has both sexes. Mated partners split up and gorge to feed their unborn, and after 2d20 days each bears one 3-foot young of 1+2 Hit Dice with every adult power, reaching full size in two months if well fed.

Xavers live on iron metals and rust. Other metals do them no harm but provide no nourishment and are later expelled, and because coins are often plated over base metal that may include iron, xavers commonly pass lumps of pure plating.

Xorn, remorhaz, fire lizards and harpies all enjoy eating them (harpies carry them skyward and let them smash on stones below), as will a few other hungry beasts. Alchemists and wizards grind xaver bodies into spell inks, unguents and alternative components for magic involving heat and electrical resistance, rusting, and protection from metal weapons. A fresh, reasonably whole adult can fetch up to 1,600 gold pieces in the right market.