Eyesen, Kala and Aydan conspired to steal one of the keys to the Conscience of Marinus, hoping to sell it. He prosecuted Ian Chesterton for the theft of the key and the murder of Eprin, but was eventually exposed by the First Doctor. He and Kala then confessed everything. (TV: The Keys of Marinus)
Behind the scenes
Eyesen's name is spelt "Eyson" in Doctor Who and the Keys of Marinus.