The Kilaaks were an alien race of silicon-based lifeforms, which all took the form of human females. They attempted to use the various kaiju of Earth to take it over in 1999. Their first two bases- one in a forest and one on the moon- were destroyed by Earth forces, and their mind-control of the monsters was destroyed.
Angered, they sent Ghidorah to destroy all the monsters, but he was soundly thrashed by a team effort on their part. Godzilla destroyed their last Earth base, and their final weapon, a Kilaak space saucer rigged to appear as a fire monster, was defeated by a team of UN astronauts. The remaining Kilaaks, who needed high levels of heat to survive, died when exposed to the colder temperatures of Earth.