We have all seen Thor: The Dark World which released in 2013, where we witness the death of Loki. It turns out that God of Lightning (Thor) and God of Mischief (Loki) are the last hope for Asgard.
The movie begins when Thor is locked away in a cage of Surtur, the fire demon. Surtur warns Thor about the end of Asgard and the arrival of Ragnarok. Thor takes it likely and frees himself from Surtur. In the process, he kills Surtur and leaves with his head for Asgard.
Upon arriving Asgard, Thor meets his father, Odin. He wants to know about Ragnarok. Thor learns that Loki has disguised himself in his father’s form, Odin. Odin’s true identity i.e. Loki is revealed. Thor and Loki come to earth to search for their father. Doctor Strange greets Thor upon their arrival on Earth. With Doctor Strange’s help them to locate their father, Odin. Odin happens to be living his last moments of his life on Earth.
Odin reveals that Thor and Loki have a sister. Her name is Hela. Odin also mentions that she is the Goddess of Death and the doom of Asgard. Shortly after Odin’s death, the sons have a fight with Hela. Hela destroys Thor’s hammer, the Mjolnir. She destroys it as if she is shattering a sheet of glass. Loki decides to activate the BIFROST by which Loki and Thor can escape. The decision came with a price. It allowed Hela to enter into Asgard and take control over the Kingdom of Asgard.
The end of Asgard
Hela manages to push Loki and Thor from her path to Asgard. They land in a place known as Sakaar. The people and things in Sakaar are banished from the nine worlds. A Valkryie, who were a group of elite women force on Asgard captures Thor and delivers him to the Grandmaster. Meanwhile, Loki becomes friends with the Grandmaster, the ruler of Sakaar.
To escape from Sakaar, Thor has to fight the never defeated champion of Sakaar. Thor agrees. Upon arrival on the battle field, Thor realises that his opponent is no other than Hulk. Thor assumes that Hulk would have mercy on him. But Hulk shows no mercy. In the end, Thor is made to lose the fight by the Grandmaster.
Thor puts together a small team which will help him escape Sakaar. They are successful in putting together a team and escaping Sakaar. When they arrive in Asgard, they evacuate the citizens of Asgard. Thor creates a distraction by fighting Hela. Thor is able to unlock his true powers. But that is not enough to defeat the Goddess of Death. They realise that the only way by which they can destroy Hela, is by destroying Asgard as her powers come from Asgard.
To destroy Asgard, they would have to bring Surtur back to life as he is the doom of Asgard. He is the one who helps in defeating Hela, the Goddess of Death.
Thor decides to take the citizens of Asgard to Earth where they can start a new life.