The eldest child of the king of Asgard, written out of its history because of what she did to build it, and released the moment her father dies.
Hela is Odin’s first child and his executioner. The two facts are the same fact. Asgard’s golden age was conquered, realm by realm, by the pair of them, and when her ambition ran past his use for it he imprisoned her and had the murals painted over.
Her return is not an invasion. It is an inheritance. She walks into the throne room, breaks Thor’s hammer with one hand, and is legally correct about all of it, which is the most uncomfortable thing about her. What she destroys in the process includes the Warriors Three and, in the end, a good part of the idea that Asgard had of itself.
Cate Blanchett plays her enjoying every minute, and the film is smart enough to let the joke and the horror share a scene.
Minor spoiler
She cannot be beaten while Asgard stands, because Asgard is what feeds her. So Thor stops trying to save the place and lets the prophecy run: Surtur destroys the realm and Hela with it, and the Asgardians leave on a ship. The film’s answer is that Asgard was never the ground.