Zim

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Zim was an overseer.

He once put the mind of Hans Vogel in the Third Doctor's body into him after attempting to wipe the Doctor's mind using a mind thresher. After he thought the Doctor's consciousness had been completely wiped, he placed Vogel's mind inside of his brain. Due to the complexity of a Time Lord mind, however, Zim was unable to completely eliminate all traces of the Doctor's consciousness. This resulted in a Meta-Crisis which threatened to destroy both the Doctor and Vogel's minds. In the end, the Doctor was able to remove Vogel from his mind by sabotaging Zim's facility. (AUDIOThe Hidden Realm)