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 stars, destroying them in the process (something reminiscent of The Fallen's plan in the movie "Transformers: Revenge of the Fallen"). As he was about to absorb Earth's sun, Devin and Max successfully stole the key for his device. Ragnarok succeeded in catching their ship, and threatened to kill Max if Devin wouldn't tell him where the key was. 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 into the Null Void, where he was taken in custody.

Ben 10: Alien Force
Ragnarok appeared in Vendetta, where he escaped from his Null Void prison and the Null Guardians 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 learned exactly who Ragnarok was 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 infiltrates his ship, and destroys the ship's command, causing the device to be damaged and the air tank to break open. Ragnarok then begs Kevin for help, stating that his father was a Plumber, honorable man and he would never have let anyone die. Instead, Kevin took the key from him and left him be sucked into cold space, seemingly causing his death.

Ben 10: Alien Force

 * Vendetta (first appearance, death)

Trivia

 * Ragnarök is the end of the universe in Norse Mythology. It’s stated that it’s where the gods and giants will die and the universe will be reborn. His solar energy manipulation and star draining might be a reference since almost all myths about their respecitve doomsdays mentions the sun desappearing or beeing consumed.