Loki Laufeyson (portrayed by Tom Hiddleston) is a recurring villain in the Marvel Cinematic Universe. Based on the character from Marvel Comics, he was the biological son of the Jotunn king Laufey, but was left to die shortly after birth. Found by Odin, he was raised by the king of Asgard and Frigga, growing up alongside his adoptive brother Thor. However, jealousy of his brother would later consume him and he plotted to humiliate and destroy Thor, thereby becoming heir to Asgard's throne. His deception would be discovered and he would fall in battle to Thor, being cast off the Bifrost Bridge.
Instead of dying in endless void, Loki fell through a wormhole and found himself at the sanctuary of the Mad Titan, Thanos. Loki and Thanos would make a deal: Thanos would grant Loki command of the Chitauri so that he could conquer Earth for himself, and in exchange, Loki would retrieve the Tesseract for Thanos. While Loki did obtain the Tesseract and summon the Chitauri to Earth, his plans for world conquest were thwarted by the newly-formed team of heroes known as the Avengers, of which his brother Thor was a member. Defeated, he was taken back to Asgard where he was to spend the rest of his life incarcerated.
However, Loki was freed from the Asgardian Dungeons by Thor when Asgard was sacked by the Dark Elves led by Malekith. During the invasion, Frigga was killed and Thor and Loki sought to avenge her and prevent the Dark Elves from using the Aether to cast the universe into darkness. During the fight against the Dark Elves in Svartalfheim, Loki faked his death, and, unbeknownst to everyone in the Kingdom, returned to Asgard and successfully removed Odin from the throne, taking his place as King where he remained for several years. During his reign, Loki sent Lady Sif on missions to Earth to investigate the attacks of Lorelei and Vin-Tak.
When Thor returned to Asgard after defeating the fire demon Surtur, he quickly discovered that his father had been replaced by Loki. The pair journeyed together to find Odin, arriving just in time to witness their father's death. The loss of Odin allowed Hela, Odin's firstborn, to return from banishment and wage war against the Nine Realms. Loki found himself stranded on Sakaar along with Thor where Loki allied himself with the Grandmaster while Thor was forced to battle against the Hulk, who had wound up on Sakaar following the Avengers' battle against Ultron. Despite trying to manipulate his way to freedom, Loki chose to side with Thor and the Revengers in a final battle against Hela, in which Loki summoned Surtur, who defeated Hela by causing Ragnarök and destroying all of Asgard.
During his final moments on Asgard, Loki took the Tesseract from Odin's vault and in the wake of the destruction of Asgard, chose to stay by his brother's side and support his ascension onto the throne. Consequences of his deal with Thanos would soon come back to haunt Loki, however, as the Mad Titan and his Black Order intercepted the Asgardians as they were making their way towards Earth. In the chaos that followed, Loki attempted to kill Thanos but ultimately failed. Taunting Thanos by telling him that he would never be a god, the Mad Titan snapped his neck, finally killing the God of Mischief.