Source: Libris Mortis: The Book of Undead
Cool Factor: A flying undead brain in a glass canister with the ability to control the living and the dead. The Brain in a Jar is great low level undead who can be the mastermind behind numerous crimes that have plagued the town. Various puppets being manipulated by this director. The Brain in a Jar is type of undead created by necromancers. It also not uncommon for those who created it to become it's own slave.
Combat: The Brain in a Jar prefers to control the situation rather then engage in combat. Most of it's abilities revolve around controlling. The Brain in a Jar is CR 4 undead opponent with the ability to fly and speak through telepathy.
-The Brain in a Jar has a few psionic abilities. It can manifest suggestion, telekinesis, and dominate person. It can also use a Mind Thrust ability that deals 2d10 points of damage on a failed will save.
-Madness: Anyone targeting a Brain in a Jar with mental spells or abilities get a backlash of madness causing 1d4 points of Wisdom damage.
-Rebuke Undead: The Brain in a Jar can turn or control undead up to it's HD.
Verdict: Besides having a great visual the Brain in Jar works well as a boss encounter. There are numerous plot hooks that can be based around this creature. A necromancer who attempted to create a Brain in Jar has become it's minion instead. Recently a thieves guild has decided to dig up the cemetery. Eventually all clues lead back to the head of the thieves guild who's strings are being pulled by the Brain in the Jar.