Hela

Odin and his firstborn, Hela, conquered the Nine Realms in a campaign of extraordinary brutality. When Odin decided to stop and rule peacefully, Hela refused. He imprisoned her, erased her from the official history, and rewrote Asgard's founding myth as one of peace. He then had two more children: Thor, and — adopted from the Frost Giants of Jotunheim after a war — Loki.

Odin dies, releasing Hela, Goddess of Death — his firstborn, and the truth about Asgard's violent founding. She shatters Mjolnir and takes Asgard. Thor is stranded on the junk planet Sakaar, ruled by the Grandmaster, where he's forced to fight Hulk, who has been there two years as a gladiator.

How was Hela imprisoned, and why? She was Odin's executioner during the conquest of the Nine Realms, and her power grew alongside Asgard itself. When Odin decided to stop conquering, she refused. He couldn't kill her, so he banished her to Hel and bound the imprisonment to his own life force — then erased her from history, painting over the murals of the conquest with images of peace. She's released the instant he dies, which is why his death opens the film.

"Worthiness" is Odin's enchantment, and Hela predates and outranks it. It's also where Thor learns his power was never in the hammer.

Thor escapes with Hulk, Loki, and Valkyrie — the last of Odin's elite guard, an alcoholic in exile after Hela slaughtered her sisters. They realise Hela draws her power from Asgard itself, so the only way to beat her is to destroy Asgard. Thor releases the fire demon Surtur and lets the realm burn. Asgard is a people, not a place.

The prophecy is Ragnarok itself: Asgard's foretold destruction by the fire demon Surtur. Thor's realisation is that it isn't a disaster to prevent but the only way to kill Hela, since her power comes from Asgard. So he deliberately triggers it.