Thousands of years ago, Odin banished his daughter Hela from Asgard and imprisoned her in Hel after her ambition to expand Asgard exceeded his.
Hela later attempted to escape from Hel, prompting Odin to send the Valkyries to stop her. She was defeated and sent back to her prison, but not before successfully overpowering the Valkyries and, with the exception of their leader Brunhilde, executing them in cold blood.
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.