Massacre of the Valkyrie

"Her power comes from Asgard. Same as yours. When it grew beyond Odin's control, she massacred everyone in the palace and tried to seize the throne. When she tried to escape her banishment, he sent the Valkyrie to fight her back."

- Valkyrie to Thor

The Massacre of the Valkyrie was the result of an attempt by the Valkyries to prevent Hela from escaping Hel.

Background
Millennia ago, Odin Borson of Asgard and his army embarked on an ambitious and violent campaign to conquer the Nine Realms. With the help of his oldest child and Chief Executioner Hela, the Allfather succeeded in his goal to create the Asgardian Empire.

It was shortly afterward, however, Odin realized Hela's ambitious nature exceeded his and was a threat to Asgard. He engaged his oldest child in battle and, after defeating her, banished her to Hel for eternity.

The Allfather then proceeded to write her daughter out of Asgard's history, ordering that any mention or depiction of her involvement in the Empire's creation be erased.

Several hundred years later, Hela attempted to escape from Hel, prompting Odin to send the Valkyries, an elite legion of female warriors who served as his special forces, to stop her.

Massacre
Riding through a portal into Hel, the Valkyries charged towards Hela on their steeds and threw their spears at the former Chief Executioner, prompting her to unleash her powers of weapon manifestation and throw innumerable Necroswords into the air. Several of the warriors and their horses were killed instantaneously while others were knocked off their horses and fell to the ground.

Hela then proceeded to overpower and kill the remaining Valkyries, with the exception of their leader Brunnhilde, who survived after one Valkyrie leaped into the path of an oncoming Necrosword.

Sometime later, Odin, who was convinced that the Valkyries perished, personally entered Hel and overpowered Hela before reimprisoning her.

Aftermath
"So... He's dead. I'd have liked to have seen that."

- Hela to Thor and Loki

With her comrades defeated and killed, Brunnhilde abandoned her title and went into exile. She eventually found herself on the planet Sakaar, where she began working for the Grandmaster as one of his bounty hunters.

Hela, meanwhile, remained trapped in Hel until 2017, when Odin's death and subsequent ascension to Valhalla allowed her to emerge from her prison.