Brain in a Jar is an element of the Dungeons & Dragons fantasy role-playing game. In addition to being undead, it is, simply put, a brain in a jar of preservative fluid. The concept may have been influenced strongly by such sci-fi and horror as They Saved Hitler's Brain and Doctor Who: The Brain of Morbius. It was introduced in Libris Mortis, a D&D supplement book that dealt specifically with undead. It is a live brain in a grimy jar filled with preservation fluid with powerful psionic abilities.