Lotor

Lotor is the resurrected "king" who wishes revenge on Voltron and Arus.

Fiction
Killed by Voltron some years prior to the start of the series, Lotor was resurrected by Maahox, using the power of Haggarium. He assumes the throne of Doom. With Maahox as his advisor, he collects Haggarium and launches several attacks against Arus and Voltron for the purpose of gathering data. He even takes on Voltron by himself, until Voltron in its Red Center formation kills him. Of course, he is revived.

He and Maahox create the Predator Robeast, a groups of five robeasts that combine into one, much like how five lions combine to Voltron. Predator Robeast was one the edge of defeating Voltron when Lotor recalled it.

Lotor and Maahox eventually developed a Haggarium-powered suit that allowed the king to turn into a giant robeast. This ability was first tested on Planet Balto, during a mission to collect data about Voltron. While Voltron defeated the Lotor robeast, the king had held on long enough for Maahox to gather a significant amount of data.

Lotor defended Caslte Doom from the Lider Robeast, during the battle he was forcibly merged with the Robeast and became the dominant personality. As Lotolider he fought the Voltron Force once again and was killed again. Maahox reluctantly revived him only so he could see his castle transform into a Robeast under Maahox's control then the planet was destroyed. It is doubtful he survived, if he didn't the only person capable of reviving him no longer has the desire to.