Kalmari

In its true form a kalmari is a drifting shape of smoky mist, like an upside-down teardrop trailing a 3-foot prehensile tail. Two unblinking yellow eyes slide across its surface to track several foes at once. Its most alarming feature is a vast, tooth-crowded mouth that seems to circle its whole body; opened wide, the jaws cover most of it. It can also inhabit a living body without changing that body's appearance.

The kalmari bites for 1d10 and lashes with its tail for 1d6. A natural 20 with the tail deals no damage but entangles the target, who may attempt a single bend bars/lift gates roll; failing that, only cutting the tail with 10 points of damage from a called shot frees them.

If a bite roll beats the number needed to hit by 4 or more (needing 12, it succeeds on a 16+), the victim is swallowed. Swallowed creatures are helpless; the creature dissolves them over rounds equal to their level or Hit Dice, after which they are lost forever and beyond raising or resurrection. The kalmari never tries to swallow a second victim until the first is dead.

It is 100% immune to spells and can be hit only by +2 or better weapons, and even those cannot truly kill it: brought to 0 hit points this way, it is merely banished to its home plane, nursing a grudge against whoever sent it back.

Its sole weakness: magic of any kind is indigestible to it, clashing with its own nature. Any magical items a swallowed victim carried are spat out the following round, and from then on those items can harm and destroy this particular kalmari regardless of enchantment. Rejected weapons gain +2 to attack and damage against it, and the spell effects of rejected wands, rods, staves and rings affect it normally, ignoring its magic resistance. A rejected wand of fire can kill it with fireballs, while fireballs from any other source still fail. These items gain no such power against any other kalmari.

Little is known of its life at home. On the Prime it is usually summoned by a mage to stand guard over a crucial place, person or object.

To stay on a world for any length of time it must inhabit a body made of native material, either a living host or an object that lets it move and show its eyes and jaws. Housed this way it can remain indefinitely; once outside a host it must claim a new one within 10 minutes or be forced home. A host keeps its own Armor Class but can absorb only 10 points of damage before the kalmari bursts out, killing it with a sound like a thousand hissing snakes. A living host must be willing, though threats, blackmail or magic can secure that consent.

Kalmari relish their power on other worlds and hate to leave, and their lawful stubbornness makes them favored guardians of mountain passes, potent magic, great treasures and secret ways. They probably reproduce by fission, though whether this can happen away from their home plane is unknown.

Records describe only one kalmari kept for long on a prime world: the pet of a wicked sorceress, lent to a crooked merchant cabal and planted in a minor functionary to seize a vital mountain pass. When a swordsman split the host open, the creature swallowed him and rejected his enchanted blade, which a band of heroes later used to slay it. The sorceress is dead, but her summoning secret may yet reach other unscrupulous wizards.