Quetzalcoatl

Quetzalcoatl was an extremely powerful being that visited Earth several hundred years earlier. During his time on Earth, Quetzalcoatl interacted with a Native American tribe in what would later become Mexico. This tribe, the Aztecs, would come to worship Quetzalcoatl as a god following his departure. While Quetzalcoatl had originally preached a message of peace to the tribe, the Aztecs used Quetzalcoatl to justify sacrificing a large number of humans over the years.

After departing Earth, Quetzalcoatl eventually came to the Klingon world of Hrakkour. Preaching a message of peace, Quetzalcoatl convinced the Klingon colonists to adopt a more peaceful outlook on life. In response, Admiral Vlict Kenka committed genocide, killing all the colonists on Hrakkour.

In 2268 the USS Enterprise (NCC-1701) encountered Quetzalcoatl on the planet Zamphor. Quetzalcoatl initially refused to believe Captain James T. Kirk when he was told of what had happened on Earth after his departure, and subjected Kirk and his landing party to several trials. Quetzalcoatl was impressed to see that Kirk and the landing party solved these trials without resorting to violence, and reconsidered what Kirk told him at his initial meeting.

Having tired of his existence of a god, Quetzalcoatl informed Doctor Leonard McCoy that there was a gland in his body that gave him his god-like powers. Quetzalcoatl, having tired of his god-like existence had McCoy surgically remove the gland, turning him into a mortal in the process.

Feeling that the level of brutality would be too much for even the Klingon government, Kenka used Quetzalcoatl as a scapegoat for the genocide on Hrakkour. The Klingons forced the Federation to bring Quetzalcoatl to Hrakkour for trial, and Admiral Kenka attempted to have Quetzalcoatl executed. An advanced being known as Bialbi intervened and stopped the execution. Bialbi would've executed Kenka, but Captain Kirk intervened and Bialbi relented - ordering Kenka to leave Hrakkour and never return on pain of death. Following this, the Enterprise returned Quetzalcoatl to his home. (Star Trek: 25th Anniversary)