The Dark One was the central protagonist of the 1987 movie Robot Holocaust.
The Dark One had a number of human air slaves which were dependent on the Dark One's machinery to provide them with oxygen, and he was able to mentally control them as well. The Dark One used his slaves to maintain his power station.
When a scientist named Jorn discovered how to break the Dark One's mental control of the air slaves, the Dark One had him kidnapped. Enveloping Jorn in a coccoon, he began to absorb the older human's body and knowledge.
Eventually Jorn's daughter Deeja and a human male named Neo along with a band of rebels confronted the Dark One, and learned that Jorn had been almost completely absorbed. Jorn begged his daughter Deeja to end his suffering, but she found she couldn't do it. Seeing Jorn in that state and realizing that with the knowledge the Dark One would gain Neo shot and killed the Dark One, which also ended the life of Jorn.
Deeja returned home to her people, determined to continue Jorn's work. With his friendship with Deeja ruined, Neo left to continue his wandering.