Chaos VS Elder Brain
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The Elder Brain was the supreme leader of the Mindflayers, a powerful being made out of the brains of many of their eldest members whose bodies were already too old to move their enormous brains. These brains converged together into a new lifeform, some sort of powerful eldritch god, a giant brain made of many smaller brains, who had his own consciousness created by the convergence of the many other consciousness that the Mindflayers had.
A powerful and terrifying being said to be able to manipulate the minds of the entire planet where it inhabited, it could easily dominate entire planets by merely reaching them, but it often remained on his hideout. Aside from his enormous mind powers, his body was frail and needed a pool of brine to maintain himself well.