The world knows Marvel's Thor thanks to his movie appearances, and comic book fans have recently seen Thor became king of Asgard. But no matter how many re-imaginings, evolutions, or adaptations may come, no story will echo like the God of Thunder and his tragic son... slain by his own hand.
Continuity can be difficult to follow in comic books, and it can especially be confusing for new readers. Because of this, Marvel Comics once had a brilliant idea to draw in a new audience without having to worry about continuity. Enter the "Ultimate" universe, an alternate continuity imprint that sought to reimagine and modernize classic Marvel characters and storylines for a new audience. The "Ultimate" universe diverged from the "main" continuity at Marvel in a couple of ways. The Avengers are called The Ultimates, Hawkeye used guns instead of a bow and arrow, and Captain America was elected president. The "Ultimate" universe ended up influencing the rest of marvel quite a bit, including the idea of Hawkeye's family from the Age of Ultron movie, and the creation of fan-favorite Spider-Man, Miles Morales. But one of the most interesting (and heartbreaking) changes in the "Ultimate" universe (that never made it onto the big screen) was the time Thor was forced to kill his supervillain son to save America from Civil War.
Modi Thorson was the son of Thor and the goddess of Death, Hela from the Ultimates timeline. The story of his conception and birth begins with the aftermath of an event called Ultimatum, which is also considered to be one of the worst event comics ever. During the event, Thor ends up sacrificing his life to save to Hela in order to save his lover Valkyrie and is subsequently forced to stay in Valhalla. While in the afterlife Hela strikes a bargain with Thor: sire her an heir, and he'll be allowed to return home. Unfortunately, this left the newborn Modi alone in the afterlife with no one to raise him but Loki, who is a lot more villainous in this universe than main continuity.
Because of this, Modi felt abandoned by his father and sought revenge. After Asgard was destroyed by the Maker (this universe's equivalent of Reed Richards who had turned evil and tried to conquer the world) With most of the Asgardians dead, Modi set out turn the United States into a new Asgard. Thor tries to stop him obviously, but it just so happens that Modi has an infinity stone on hand to brainwash Thor into becoming his minion. Modi's plan is to manipulate different factions in America into going to war with one another so that the US is divided and he can take over instead. The plan would have worked too, except Captain America and the Ultimates were there to fight him.
The climax of this story took place in Ultimate Comics Ultimates #18 where Thor manages to break free from Modi's control just as the Ultimates defeat his army of Hyrda goons. With no more tricks up his sleeve, Modi offers Thor a grim warning before being killed: "You will pay for striking down your only Son." It's a dark moment, but ultimately the god of Thunder does what is necessary to end the fighting. Thor then offers one final apology to Modi posthumously: "For all I have done, and for all I have failed to do--I am sorry, my son." This line finally brings the war Modi started to an end, and it also marks one of the hardest decisions Thor has had to make in the "Ultimate," or any other universe.