Kyomu

Kyomu (虚無 Nihility, Nothingness) is an ancient creation, and the possibly the most powerful force in the known Bleach universe.

History of the Kyomu
Thousands of years ago, long before the Department of Research and development was created, a few members of the Soul Society began to conduct an experiment, trying to increase their own power. They had decent enough spiritual power, but were interested in gaining more power. However, their experiment soon went awry, giving birth to Kyomu. Kyomu possessed one of its creators, and the remaining two fought back with their spiritual powers, a choice that proved fatal on their part, as the possessed absorbed their attacks, and then killed them with it. However, before the possessed man could go on to kill anyone else, multiple powerful residents from the Soul Society managed to extract it from him. Releasing the danger that such a creation posed, they managed to contain it inside a barrier, and sealed it away inside a different dimension, placing guards over it.

Abilities of Kyomu
Kyomu can absorb all forms of power that it comes in contact with, even being able to take in reason. It is sealed inside a barrier, and, when released, takes the form of dark smoke. It enters powerful beings such as Shinigami or Arrancar, possessing their physical form. It cannot be destroyed or harmed by any known means, rather it can only be contained by spiritual beings of high power. It actually possesses no mind or personality of it's own, merely consuming all power it touches. Kyomu's only desire is to absorb power, and if anyone, even a human, is possessed by it, they too will crave power like it does, until they eventually destroy everything around them. Kyomu is sealed away inside the pocket dimension similar to the one the Spirit King is located in. Kyomu manages to "remember" every power is has ever absorbed, so even when taken in by someone, if it is cast out, the powers it absorbed never leave it, and are able to be used by the next person it possesses.

Trivia

 * Kyomu is based off of The Hollow from the Charmed series.