This is rather freely adapted from The Villain's Lorebook, a free download from WotC. It's based on a creature in the novel Azure Bonds.

Hit Dice: 3d8+3 (15 hit points)

Initiative: +0

Speed: 20 ft., Fly 20 ft.(average)

AC: 26 (+16 natural)

Attacks: Bite +1, Whip +1

Damage: Bite 1d10+1; Tail 1d6+1

Face/Reach: 5 ft. by 5 ft./5 ft.

Special Attacks: Swallow, Entangle

Special Qualities: Damage Reduction 15/+2,

Saves: Fort +4, Ref +3, Will +4

Abilities: Str 13, Dex 10, Con 13,

Int 9, Wis 13, Cha 7

Skills: Bluff +5, Intimidate +2, Intuit Direction +4,

Appraise +8

Feats: Power Attack

Climate/Terrain: Any

Organization: Solitary

Challenge Rating: 3

Treasure: Double Standard

Alignment: Lawful Evil

Advancement: 4-6 HD (Medium)

7-9 HD (Large)

Kalmari are creatures native to Thuldanin, the second layer of the plane of Acheron. In their natural forms, they seem to be thick, vile clouds with enormous tooth-filled maws, a sinuous, prehensile tail, and two unblinking yellow eyes that move about the surface of their amorphous bodies as they track their foes.

Kalmari are rarely found in that form, however. More commonly, they inhabit another living creature's body, or a non-living body built with places for the aberration to articulate its jaws and tail. In its native layer it will appear as a bizarre conglomeration of trash and spare parts, biting at its opponents with teeth made of nails and torn metal and lashing out at them with tails of braided power cables and discarded chain mail.

Combat

Kalmari attack viciously and without remorse,

Entangle (Ex): When a natural 20 results from the tail's attack roll, no damage is done but the victim is entangled. An entangled victim is allowed one strength check (DC 13) to escape; otherwise a successful disarm action can force the kalmari to let go. If everything else fails, only severing the tail itself by inflicting 10 points of damage there will free the entangled victim.

Swallow (Ex): The kalmari's most fearsome ability is that, on any attack roll (for the kalmari's mouth) that is 4 or more higher than the AC it would need to hit that opponent, the character is swallowed whole. For example, if the kalmari was attacking a character with an AC of 12, any natural roll of 16 or greater would mean that the target had been swallowed whole by the creature. Swallowed creatures will be digested in a number of rounds equal to their experience level or hit dice. Swallowed victims are helpless. After this time, the victim is gone forever and cannot be raised or ressurected, though wish will still work. The kalmari will not attempt to swallow a second target until the victim is dead.

Host Body Vulnerability (Ex): A host body retains its normal Armor Class (which is usually significantly less than that of the kalmari itself), but can withstand only 10 points of damage before it is rent asunder as the kalmari erupts from it, killing the host. The noise made by an emerging kalmari is that of a thousand snakes hissing at once. A living or sentient target of a kalmari's possession must be willing, though that decision could be coerced (through force, blackmail, or magic). A nonliving, nonsentient target can be posessed automatically as a free action.

Outside Acheron, kalmari without a host made of native materials must find one within 10 minutes or be banished back to their home plane.

Immunity to Death (Su): While physical weapons and even magic can occasionally damage the beast, they cannot kill it. If reduced to 0 hp by such weapons, the kalmari is merely imprisoned within a nearby inanimate object, which it will attack and destroy when it has healed back. Off-plane, kalmari reduced to 0 hp or less are banished back to Acheron. In either case, kalmari harbor grudges.

All magical items worn or carried by a victim the kalmari has swallowed are regurgitated by the beast the round after ingestion. Offensive magical items such as weapons, wands, staves, and so on are then capable of damaging and destroying the beast regardless of their enchantment. Weapons are granted an additional +2 attack and damage bonus vs. the creature and the spell-like abilities of wands, rods, staves, rings, and so on now inflict damage on the kalmari normally: the beast's Spell Resistance and Damage Reduction no longer applies to those items. If, for example, the kalmari ingests and rejects a wand of fire, fireballs from that item can kill the kalmari. Fireballs from any source other than that wand are still subject to the creature's normal spell resistance. Note that any items swallowed and rejected by one kalmari do not gain these benefits against any other members of the species.

Rarely, a kalmari may even treat such a rejected weapon as its completion, its carefully guarded charge (see below).

Kalmari Society

It is thought that originally the kalmari lived exclusively in the void between Thuldanin's cubic continents, hunting birds and diving down to a surface to gather carrion. Over time they developed an affinity for the cast-off junk and weaponry littering Thuldanin's surface, and this affinity became the addiction the kalmari call completion. The creatures choose an interesting-looking relic or war machine of the sort that naturally collects on their native layer and guard it fanatically, creating a solid form nearby in order to better relate to their charge. For a kalmari, life without such a duty is empty, unbearable, and incomplete. As a result of this peculiarity, other races have found the creatures make excellent guardians. The secret of summoning a kalmari has even been discovered by a few conjurers on the material plane. Kalmari on planes other than Acheron must have a living or non-living body composed of material native to the plane.

A kalmari within a host body can exist there indefinitely. A kalmari is very reluctant to leave its charge. Even when they feel the urge to reproduce (about once a year), they do it near (or ocassionally in) the object that completes them, splitting off a small part of its substance and inserting it into a nearby object, where it gestates for 1-2 years, emerging as an adult kalmari. A kalmari will battle with its parent for the right to be completed by the same charge; usually the inexperienced young one is driven off to find a charge of its own.

Kalmari cannot reproduce outside Acheron.

Variants

In Avalas, the first layer of Acheron, feuding factions build combat machines especially designed to house kalmari. Because of their greater quality, they do an extra point of piercing or blugeoning damage per attack.

The bladeling conjurers of Ocanthus summon kalmari into even more deadly devices, whose whirling knives cause an additional 1d4 points of slashing or piercing damage for every attack they make as long as the hosts last.

Occasionally wizards of Acheron's other layer, Tintibulus, will summon kalmari to guard their spells and treasures. These usually end up with standard statistics, unless their hosts are enchanted. In Tintibulus, they most often use living spell-bound slaves, rather than machines