The container is always what made it "innocuous but creepy" upon first discovery, until it starts throwing things around or "mind controlling" people. In the end this needs to be a Mage Wars Brain in a Jar, and not some other game's Brain in a Jar. What kinds of flair make this uniquely Etherian?
Well, flavor wise, there are lots of Mage Wars -esque ways a “Brain in a Jar” could play out. Here are some territories to consider:
Draconian BrainThis could be the brain of one of the original Dragons, ripped from its host and enslaved in an jar-like artifact during the Elemental Wars. After so many hundreds of years of imprisonment and disembodied torture, it has become a dark and twisted thing.
Infernian BrainThis could be the brain of a shadow demon. Shadow demons are minions of Moloch, Lord of Curses. The brain may have some sort of ability to project curses psychically.
Desjer-TetPerhaps this is the brain of a Djinn, enslaved in a crystal lamp. After all, the people of Desjer-Tet favor Dark magic, and their people are infatuated with life after death, and the world of the undead. There is also considerable emphasis there on the practice of Mind Magic, and of using its powers to help move great stones and build mighty monuments to the Dark Gods.
KumanjaroOr this could be a creature/relic of the mysteries of Kumanjaro. There are six tribes of peolple living in the jungles of Kumanjaro, but one tribe, the Histanti is a complete mystery. The Histanti disappeared several centuries ago, leaving nothing behind but well preserved stone cities. The other five tribes refuse to speak about what happened; only that the disappearance happened in the span of a single night, and every Histanti was gone the next day. The Histanti is not their proper name, but actually a Kume word meaning “Adversary”. One missionary who befriended a tribe of Kume asked what the Histanti real name was, and was told it was not permitted to say “lest they hear you”. All five tribe share taboos about the Histanti, leading some scholars to wonder whether the five surviving tribes are loathe to speak of the Histanti out of fear, or guilt. This could be the brain of the last of the Histanti, shackled and enslaved.