Thousands of years ago, Odin banished his daughter Hela from Asgard and imprisoned her in Hel after she attempted to take over Asgard.
Hela later attempted to escape from Hel, prompting Odin to send the Valkyries to stop her. She successfully overpowered the Valkyries and, with the exception of their leader Brunhilde, executing them in cold blood, but before she could escape, Odin personally entered Hel and defeated and send her back to her prison.
After the death of Odin, his power over Hela's imprisonment waned until there was no force keeping her at bay. She stepped out of a portal-like gateway soon after, musing that she would have liked to see her father die.