The Monster of Peladon
Eckersley, along with Vega Nexos, was a Galactic Federation mining engineer stationed on Peladon.
Eckersley was secretly working for Galaxy 5 to subvert trisilicate production and send it to Galaxy 5 instead of the Federation which desperately needed the mineral for their technology. He used a heat ray disguised as the spirit of Aggedor to kill miners and frighten the rest into ceasing work.
He convinced Ambassador Alpha Centauri to call for the Ice Warriors led by Azaxyr (who, unbeknownst to the ambassador, were also working for Galaxy 5) to take over as a show of force to make the miners work when they were beginning to rebel. Eckersley would have gained a cut of the profits from the trisilicate sent to Galaxy 5.
He planned to rule Earth after Galaxy 5 had defeated the Federation, only for his and Azaxyr’s plans to be exposed by the Third Doctor. He took Queen Thalira hostage and fled into the mines, where he encountered and was killed by the real Aggedor — but not before he had fatally injured the creature. (The Monster of Peladon)