Krell

The Krell were natives of the planet Altair 4 in the film Forbidden Planet. They died out when they completed their project to gain freedom from "instrumentality" - freedom from corporeal existence. Unfortunately, the 20-mile-cubic machine they created for this purpose gave physical form to all of their unconscious hatreds of one another; unfamiliar emotions that had seemingly been eradicated from their society killed them all in the space of a single night. The unstoppable machine committed genocide.

No image of what the Krell looked liked survived, but the human who studied them, Dr. Edward Morbius, theorised that they must have had triangular body-frames and very large heads.

The Krell were highly intelligent and cultured, having even visited Earth long before Man had even become tool-users. Unfortunately, eons of peaceful existence made them forget their baser selves, which ultimately destroyed a great civilisation as it was about to achieve its apex.