The Krell are an extinct race of advanced beings of the planet Altair IV in Forbidden Planet. A human called Dr. Morbius was the only researcher to ever study the extinct Krell.
The Krell had reached an incredibly advanced stage of technological and scientific development, able to reproduce matter down to the nuclear level. They were able to construct a 33,000 km3 underground machine that could turn thoughts into reality and project that anywhere on the planet, giving a physical form and life to their Id.
Dr. Morbius used their plastic educator, a device able to create a 3d visualization of the operator's thoughts, to boost his IQ such that he could understand some of the basic science of the Krell. Morbius believed that the Krell, after millions of years of social advancement, were not only technologically but morally superior to humanity—a benevolent and noble race of scholars and pacifists who had evolved beyond war and violence.
Morbius eventually discovered that in a single day and night, 2,000 centuries ago, the entire Krell race disappeared. All above-ground evidence of their civilization had vanished. When combined with the power of their vast machine, the unbridled emotions of their Ids were let loose to eradicate the entire Krell race.