Aggedor

Aggedor is a semi-mythical sacred beast of the planet Peladon. He was used by the high priest Hepesh to destroy his enemies, but he was tamed by the Doctor. Aggedor was killed by Eckersley who died himself while he was trying to kill Aggedor.

Hepesh (the high priest during the Doctor's first visit)  feared that if the planet joint the Federation it would change the planet and he would be found out as a traitor. Aggedor also killed him.

Eckersley was a traitorous mining engineer from Earth who worked with the Ice Warriors and used deadly projections of Aggedor to halt the work in the trisilicate mines of Peladon.

Aggedor was in The Curse of Peladon and The Monster of Peladon