Although the beasts were long believed dead, High Priest Hepesh located one and trained it to kill on command. He sought to destroy Peladon's links with the Galactic Federation and keep Peladon pure in its beliefs and heritage.

The Doctor soothed Aggedor with gentle hypnotism and a Venusian lullaby, and used it to expose Hepesh. Aggedor slew Hepesh and was then kept safely below the Temple of Aggedor beneath the citadel.

When, fifty years later, the human engineer and traitor to the Federation, Eckersley, held Queen Thalira hostage, the Doctor used Aggedor to track them both. Aggedor attacked and killed Eckersley, who shot the beast dead at the same time.

Eckersley had been maintaining a reign of terror amongst the Pel miners by creating a holographic graven image of Aggedor with a built in heat ray. After its death, Aggedor still remained the sacred symbol of the planet, reminding Peladon of its heritage at all time