Ragnarok

Ragnarok is a character introduced in season 3 of Ben 10: Alien Force. He is notable for being the one who murdered Devin Levin, Kevin's father.

Backstory
According to Max's flashback, Ragnarok was originally a powerful and dangerous alien criminal from unknown origin, who possessed a ship able to drain energy from suns, destroying them in the process(something reminiscent of the Fallen's plan in the movie "Transformers 2: Revenge of the Fallen"). As he was about to absorb Earth's sun, Devin Levin and Max Tennyson successfuly stole the key for his device. Ragnarok succeeded in caught their ship, and threatened to kill Max if Devin wouldn't tell him where was the key. When Devin declined, Ragnarok attempted to kill Max, but Devin put himself between them, and was fatally wounded instead of Max. Furious, Max used the element of surprise to send Ragnarok in the Null Void, where he was taken in custody.

Alien Force
Ragnarok appeared in Vendetta, where he escaped from his Null Void prison and arrived on Earth. After he trashed Kevin's house in order to find his key, Kevin's mother revealed him what exactly happened to his father, causing the young Osmosian to start looking for Ragnarok in order to get revenge. Feeling worried for Kevin, Ben and Gwen learnt who was Ragnarok from Max, and attempted to help Kevin, only to be asked by him to stay out of his way. Eventually, Ragnarok got the key back, and attempted to use it to activate his machine, but Kevin successfully infiltrate his ship, and destroy the ship's command, causing the device to be damaged and the air tank to break open. Ragnarok then beg Kevin for help, stating that his father was a Plumber and would never have let him died. Instead, Kevin just took the key from him and left him be sucked into cold space, seemingly causing his death.

Ultimate Alien
It's currently unknown if Ragnarok will appear again, though Dwayne McDuffie stated he believed it would be a good idea.

Abilities

 * Superhuman Strength, Stamina, and Durability
 * Solar Energy Manipulation
 * Agility

Trivia

 * Ragnarok is the end of the universe in Norse Mythology, where the gods and giants will die and the universe will be reborn.
 * He is one of a few Ben 10 villains known to have commited murder/genocide(the others being Vilgax, Aggregor, and Urian). Because he was responsible for killing Devin Levin, he is Kevin's sworn arch nemesis(basically he is Kevin's Vilgax).
 * Though it is most likely just a coincedince he shares many physical charictaristics with Doomsday of DC fame who at a time had coarse White hair and grey skin. It also may be noted that there names share the same meaning "the end of the world."
 * Ragnarok used to have a son before he died; his name is Outbreak and he will probrably appear in a future episode.
 * It is possible that Ragnarok and Sundur could be of the same alien species, as they both have white hair and gray hair and they both wear red and black clothes(that could just be a coincidence though).