These were taken from the Planescape Doors to the Unknown module, where they were given much shorter descriptions and intended to quickly die off. But several of my players are into creepy body horror bioware and one is playing a Great Old One warlock, so I went ahead and expanded the descriptions to figure out how to keep them alive and give them more versatility. Taken all together, they paint a lovely picture of gear for a sahuagin or deep one assassin/infiltrator.

Not actually magical, these items are composed of a steel-hard shell of colorful coral with an internal biological system that can sprout cilia, crawl somewhat into position, and attach to a living host. They derive most nutrition from this symbiotic relationship, though the host and coral will be healthier with regular intake of seafood. They will also slowly deteriorate if not soaked in seawater for at least an hour a week (a soaked rag placed around the item will do), or periodically fed powdered limestone (or similar material like pearls). If damaged, they heal naturally with their host (undoing the broken condition after a long rest). They can be magically enchanted like the items that they are (though they would lose the enchantments if allowed to die). Their lifespan is unknown.

They can be coaxed to detach from a host with brimstone-based smelling salts, but otherwise must be violently ripped or cut free, dealing major damage to the host (and likely killing the item).

Carapace Breastplate

This large shell wraps the entire torso and shoulders in a thin and articulated but dense shell.

It functions as breastplate armor with no effective weight/encumbrance. It can be worn under clothes (though likely of a larger size than normally required).

If soaked in a solution of various expensive poisons carefully and gradually, it might develop an immunity to toxins that it can pass to its host, granting advantage on poison saves (from poison-coated weapons) and resistance to poison damage.

Claw Grapple

This item is an articulated three-fingered claw attached to a flexible, ropelike tendril. The rope attaches to the back of the wrist then compresses when not in use, the claw laying against the back of the hand. This can make bracers or the equivalent uncomfortable, but does not interfere with shields.

It can be launched to grab protrusions up to ten feet away. This can be used to snag small items at this range or to latch onto the object to yank or swing (and might require an attack roll to target, at the GM’s option). You have advantage on climbing checks and cannot fall more than ten feet (this may not be possible if the surface is extremely smooth and there are no possible grapple holds within range). You can make a grapple check at its range, though it simply tethers you to the target rather than restraining the target (the grapple releases normally with a contested grapple check, or if the target deals 4 or more damage to the rope, giving it the broken condition).

If carefully built up with a paste of high-quality pearls, the claws might be extended long enough that they can be used as a punch dagger, as needed, including striking at the grapple’s range.

Face Shell

This item is a smooth, blank shell the perfect shape to fit over a humanoid’s face and, in fact, happily binds itself like a mask if so placed. The host does not have any problem seeing or breathing, but needs to remove the shell to eat. Unlike the other devices, it seems a little easier to coax into releasing so its host can consume food, and can even be trained to move to sit on a shoulder when not in use as a mask.

The shell grants you darkvision to 30 feet, or 120 feet in saltwater. Additionally, it allows you to breathe water (or air, if you normally breathe water). Finally, it filters inhaled toxins, giving you advantage on saving throws against inhaled poisons or airborne diseases. However, due to its unnerving blank visage (the only sign of the eyes are slightly more vivid circles of color), you have disadvantage on any Charisma checks relying on generating positive feelings (e.g., intimidation is not affected).

A potentially unnerving feature of the shell is that it could be placed on the face of a corpse submerged in seawater and powdered hermit crab shells, gradually turning into a death mask of the subject as the body decays. You subsequently gain advantage on Charisma (Deception) rolls and similar rolls to masquerade as the subject using disguise self, alter self, or similar form-concealing magic. The mask helps mimic the subject’s voice and mannerisms. It can only retain one such subject at a time.

Mantis Blade

When detached, this looks like a long, thin sword blade. It can be attached behind the wrist, at which point it hinges back when not in use, resting against the back of the forearm. This makes it challenging to wear anything else on that arm, such as a bracer, shield, or even tight sleeves.

It functions as a rapier (but can instead act as a shortsword if you are not proficient in rapier or want to use it as an offhand weapon). It also works as a prybar, and has enough dexterity to be bent as desired to manipulate objects as would make sense for its structure.

If soaked in a bath of seawater and powdered silver, iron, or adamantine, it could conceivably take enough of those metals in to strike creatures harmed only by those materials as if made of that material.

Reservoir Valve

This item is a roughly cylindrical shell around the size of a large man’s fist. When attached, it flattens somewhat and tries to find a location on the back where it will not be an impediment.

The valve contains a naturally-generated healing potion. It can contain one charge at a time, and regains this charge when you take a long rest that you begin with both full hp and hit dice (i.e., it absorbs your unneeded natural healing). It can also have a charge restored by casting two levels of healing magic into it (e.g., either two first-level cure wounds or one cast at second-level). You can expend the charge as a bonus action to have it inject you with the potion so you heal 2d4+2 hp. Additionally, if you are reduced to 0 hp, the valve automatically expends its charge, if available, to heal you at the beginning of your next turn, before you have to make a death saving throw.

If bathed in a solution of several higher-powered healing potions as well as a high-quality oceanic chemicals, it might have the power of its healing solution improved to more closely mimic these better potions.