The Omnethoth was a sentient weapons system designed by a race called the Masters, who created it with universal domination in mind.
The Euterpians and Greld used the Omnethoth and the battle shroud of the Semquess during the Millennium War, destroying an entire galaxy to destroy a thousand of the Mad Mind of Bophemeral's fleets. (PROSE: The Quantum Archangel)
In 2992, the Omnethoth eventually destroyed the Masters and then found its way to the Minerva system, where it totally destroyed Yquatine. It appeared in orbit as a black cloud, and it released acid rain into the atmosphere, killing all life and destroying all traces of civilisation. The Eighth Doctor reprogrammed the Omenthoth to make them tame and harmless, after which President Vargeld of Yquatine had them destroyed to prevent them being a threat again. (PROSE: The Fall of Yquatine)