The Zircon were a species led by an Emperor. After conquering and "finish[ing] with" one planet, they would move to another.

They created a galactic cyclone by destroying a dying planet and directed it to Earth. The toxic gases in the cyclone would have destroyed most life on Earth, allowing the Zirconians to invade with minimal opposition.

In one account, they were stopped by the Third Doctor and Sarah Jane Smith, who warned Alistair Gordon Lethbridge-Stewart about the cloud. In another, the Fourth Doctor and Joan Brown were the ones to stop their plans, having warned Maxwell-Lennon.

Their fleet was apparently destroyed when the Doctor destroyed the poison cloud. (COMIC: Doomcloud)

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