From Halopedia, the Halo wiki
Pluton was created to counter the strength of the Spartans. He was sent to the planet Cronkee by the Prophet of Truth to kill the Master Chief but ended up fighting SPARTAN-1337 instead. After some chaotic fighting Mama opened a slipspace portal via the crashed frigate she resided in and fired its magnetic accelerator cannon at Pluton, sending him through the slipspace portal. Pluton was last seen with his head trapped in the MAC slug and drifting towards a Halo ring.
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