King Xarion, also known as King Viktor, was the monarch of Zarkovia.
Ben 10: Ultimate Alien
King Xarion contacted Ben Tennyson to help him to stop his son Prince Gyula from using the brain-dead body of Dr. Viktor. After Ben and his team were defeated, he was overthrown by his son and thrown into the dungeon. Gwen then let him go, but he betrayed her and kidnapped Ben for experimentation. He did when his son shut off his machine's power as he was hooked up to a mind-transfer device, albeit simply transferring his mind to Dr. Viktor's body, making him King Viktor. Smaller size Vicktor.
King Xarion was later beaten by Ben, Gwen, and Kevin but could not be arrested seeing as actions within that country undertaken by humans are regarded as out of Plumber jurisdiction (regardless of the fact that Xarion had been inhabiting an alien criminal's body).
Ben 10: Omniverse
It was revealed in Rad Monster Party that Zs'Skayr eventually entered Dr. Viktor's molten-hardened body and devoured King Xarion's soul, thus reviving the original Dr. Viktor.