The Unfettered Mind

Through an eldritch ritual combining alchemy, necromancy, and surgical precision, the brain of a mortal being is encased in a glass canister filled with alchemical preservative fluids. The brain is rendered immortal and imbued with psionic powers so that it can spend eternity plotting and executing its desires, even casting spells and emitting psionic energy that can cause great harm to others in the vicinity.
The longer a brain spends divorced room its body, the more likely some form of insanity will take over it. The brain becomes shrunken and wrinkled in undeath, and the jar can grew grimy with time. In all, brain, jar, and fluid typically weighed around 25 pounds (11 kilograms)

Creation

A masterwork of the art of preservation of biological matter, this process is both alchemy and necromancy, maintaining life in a state of undeath. It is not sufficient to pluck the brain of a zombie or other undead and put it in preservatives, as it would still lack thought and psionic power. Instead, creation of a true disembodied brain requires a specific extraction ritual, spells of formulation, and recipes of alchemical preservation, all of which are well-kept secrets known to a select few master necromancers.
It is difficult and hazardous process, for as its powers grow, the brain is apt to dominate the mind of the necromancer foolish enough to create it.

Personality

No longer obliged to control an entire body, the brain acquires great powers of the mind. They often take control of the minds and bodies of other beings, particularly the living.
A disembodied brain is particularly susceptible to dementia, schizophrenia, and paranoia, often twisted toward evil.

History

The first known successful extraction occurred after the star elves of Sildëyuir executed the corrupt wizard Mourel Duskwalker, his apprentice Kyjal Stardancer retrieved his head. Mourel had been experimenting with a certain method and Kyjal used it to extract his master's brain, which he placed in a jar of embalming fluid. Then, using another of Mourel's techniques, Kyjal animated the brain. For defense and mobility, Mourel had a compartment cut into the chest of a commanded girallon zombie that could carry his jar. Over a century as a brain in a jar, Mourel evolved into a greater kind of undead, but he, Kyjal, and their minions remained trapped in the citadel of Tir'in'tiral sealed away in the Night Realm.