The Ergon was a creature created by Omega as one of his agents in the universe of matter to act on his behalf.
Biography
Omega created the Ergon with his power of psychosynthesis. He considered it one of his less successful creations. (TV: Arc of Infinity) Bizarrely, while on Earth (presumably in Perivale, where she grew up), Ace saw it while working at a fast food restaurant. Ace, at the time, mistook it for a person wearing a costume. (PROSE: Anti-Matter with Fries)
It used a matter conversion gun, with which it shot Colin Frazer, Tegan Jovanka and Robin Stuart. While wrestling with the Fifth Doctor, Nyssa took its gun and shot it. (TV: Arc of Infinity)