In 2269, the USS Enterprise discovered their homeworld, and Lucien was the first to greet them. He did so, however, under the condition (and the ruse) that the other Megans would not learn of the Humans' presence at Megas-Tu.
Following the discovery of the Human presence, the Enterprise crew was placed on a Salem-type trial by Asmodeus to answer for the crimes of their forefathers. Lucien himself was given a mock trial for welcoming the Humans to their planet. The Humans seemingly won their trial, but the Megans were unconvinced the Enterprise's historical records weren't faked. In order to test the Enterprise crew further, Asmodeus pretended to sentence Lucian to an eternity in limbo. When Kirk came to Lucian's defense, Asmodeus claimed Lucian was known by another name on Earth, Lucifer.
Kirk remained willing to sacrifice himself to save Lucien, despite his previous aliases and reputation from his time on Earth. In doing so, Kirk proved to the Megans that Humans had changed over the past six centuries. The Megans informed them that they had passed the Megans' test and welcomed them to their planet with open arms.
The voice of Lucien was provided by James Doohan.
In the script for "The Magicks of Megas-Tu", Lucien is initially described as "an alien, half-goat, half-man, complete with cloven hooves, horns, and tail. He is the image of all the goat gods of supernatural mythology, bearded and broad, with a strange red glint in his eyes. He looks about fifty years old, and there is a wonderful vigor about him." He is also said to have "a voice like thunder" and the script later refers to him once as "the goat-man."
Despite being consistently named Lucien in the script, he at first introduces himself to the Enterprise crew as Baal, though this line of dialogue is not in the episode's final version. Another ultimately-unused line, spoken by Lucien during the trial, establishes that he visited Babylonia, Mesopotamia, and Greece while on Earth.
Lucien's character design appears very similar to that of a satyr boy who appears in "The Magicks of Megas-Tu". Also, the novelization of that episode (which was published in Star Trek Log 3 and repeatedly refers to Lucien to as "the goat-man") refers to the satyr child (who is referred to as a "satyr" only in the episode's script) as "a small goat-boy," indirectly acknowledging the similarities between their looks.