Beyond the world you know, there exists the realm eternal, Asgard. The shining light of the nine realms. All of its inhabitants bow down to the All Father: Odin.
Unfortunately, most outside of Asgard do not agree with Asgard's diplomacy and untold power over Midgard. Due to Asgard's lack of intervention in maintaining peace within the other realms besides Midgard, the other realms have fallen into chaos. Without order in the nine realms, Odin worries that Ragnarok is upon Asgard, and his beloved kingdom shall perish. With that in mind, Odin orders martial law within the disordered realms-noone can enter or leave. And to distill this chaos, Odin, against the orders of Asgard's high council, sends in his most valuable asset and beloved treasure: his son, Thor.
Thor notices something odd about the realm: it is quiet. Too quiet. Thor looks around, and notices that all of the houses and castles are completely intact, yet noone is around. Not even the animals of the realm exist. Mjolnir then detects a unique energy signature in the realm "mystical energy heading towards this area." Thor looks up and creates a force field which shields him the blast only a mere moment before it makes contact with Thor. The blast is so powerful it destroys the entire city Thor was traveling through. "Odin's beard, what have at thee?" Thor looks at what caused the blast, it is a silver sword, that sparkles in sunlight, and Thor realizing the horror, says "thy, it cannot be, tis sword that shines brighter than the star in the sky can only have been made in one area, Olympus."
By the time Thor drops his force field down, the sword returns to a dark elf, some 6 feet tall, with black skin, hooves as feet, and horns that make the elf look like a devil. "Thy is the Mighty Thor! come to bring more pain to me and destruction to this realm! Ahhhh!" The elf throws the Olympian forged sword at Thor, hoping the sword kills Thor in one shot. A slicing noise is heard, blood drips on the ground, both look at each other, with painful looks on their face. As Thor looks down, the Elf falls dead, from an a knife thrown at it from behind, from an unknown assailant.
Thor finds out that it is Ares, the olympian god of war, in Asgardian territory, a direct violation of intergalactic law and a promise of war. But instead of panicking, Thor decides to resurrect the elf, preventing its soul from being claimed by Hela, and follows Ares into the forest. Unknown to Thor, ares was prepared, the moment Thor walks in, a magical asgardian gas(only asgardian magic can harm thor) causes Thor to black out, with his hammer at his waist.
He finds Ares and kills him and his followers. Then he travels to the other nine realms and kills off all of the terrorists, stabilizing the nine realms and putting Asgard back into power.