Chaos Beast

Nightmarish Blob. Foul and terrible, this human-sized mass of flesh has no set form. A chaos beast constantly melts and reforms, apparently drawing each shape from every nightmare that has ever plagued every thinking creature. It chaotically shifts through a dozen monstrous forms. There’s no telling how a chaos beast will look. One moment it might be a towering horror of hooks and fangs, all pulpy flesh and exposed veins, and the next a slithering mass of ropy, vermilion-tipped tentacles. Then it becomes a mighty creature, all muscle, and fury. A chaos beast’s dimensions vary, but medium-sized ones usually weigh about 200 pounds of warping flesh and bone.

Infected by Chaos. The first encounter with the beast has was theorized to have concurred around the same time as the inception of the Gibbering Mouther. Chaos beasts are often the byproduct of the foolish or insane or were innocents onset by the truly wicked. Humanoids exposed to chaotic energies of the Far Relm, Limbo, the Realm of Dreams, the Elemental chaos, or even too much radiant energy may quickly devolve a person into a chaos beast. They lose all sapience as they begin to lash out against everything because of the unbearable pain. 

Shapeless Disease. For all its fearsome appearances, whether it has claws, fangs, pincers, tentacles, or spines, a chaos beast does little physical harm. Regardless of form, the creature seems able only to muster a few meager attacks as its continual transmutations undermine the hitting power of their temporary appendages. The true danger of this creature comes from its ability to pass along its foul condition, Corporeal Instability,  to other creatures with a swipe.

Profanes Uses. Because of the madness and pain, the chaos beasts will lash out at anything that gets near it, making it difficult to control, and must be held in isolation. Evil creatures may inflict Corporeal Instability on a person as a form of horrific execution to send a message. Demons and Slaadi may sometimes teleport in a few to sow confusion in a bastion of order as a prelude to their assault. 

Corporeal Instability.

Transmited to creatures from a blow from a chaos beast can cause a terrible transformation, quickly becoming a spongy, amorphous mass. Unless controlled through an act of will, the victim's shape melts, flows, writhes, and boils befor lossing thier mind completely and becomeing anouther chaos beast.

The affected creature is unable to hold or use any item. Clothing, armor, rings, helmets, and backpacks become useless. Large items - armor, backpacks, even shirts - hamper more than help, reducing the creature's Dexterity score by 4. Soft or misshapen feet and legs reduce speed to 10 feet or one-quarter normal, whichever is less.

Searing pain courses along the nerves so strong that the creature cannot act coherently. It cannot cast spells or use magic items, and it attacks blindly with unable to distinguish friend from foe, regarding all creatures it can see as enemies. Whenever the affected creature chooses another creature as a target, it must choose the target at random from among the creatures it can see within range of the attack, or other ability it’s using. If an enemy provokes an opportunity attack from the affected creature, the creature must make that attack if it is able to. All attacks the affected creture makes are with  disadvantage.

At the end of each round the creature spends in this amorphous state reduces its Wisdom score by 1. If the creature's Wisdom score falls to 0, it becomes a chaos beast under the DM’s control. At this point only a wish spell can reverse the transformation and the effects of Corporeal Instability.

A creature with a strong sense of self can regain its own shape by using their  action to attempt a DC 15 Charisma check. A success reestablishes the creature's normal form for 1 minute, supressing the penaltes caused by Corporeal Instability and the reduction of thier Wisdom score at the end of each of thier round but don't restore points of wisdom that were lost.

Corporeal instability is not a disease or a curse and so is hard to remove. A shapechange or stoneskin spell does not cure the afflicted creature but fixes its form for the duration of the spell. A lesser restoration, heal, or greater restoration spell removes the affliction and restores all lost wisdom.

A creature found in the book Monster Manual for D&D 3.5, adapted to 5th Edition.