Answer: Asgard must be destroyed
Thor, 142, and Banner return to Asgard just as Hela and Skurge flush out Heimdall to reclaim the Bifrost sword. Thor splits up from the other two in an attempt to locate Hela who, out in the mountains, locates Heimdall and the hidden Asgardians fleeing their safe haven. She only leaves their trail when Thor summons her from the throne, giving the last opportunity for the citizens to take the Bifrost Bridge to safety. Unfortunately, the great wolf, Fenris, and Skurge's forces flank them on the bridge, only avoiding fighting the untrained people of Asgard when 142 and Banner swoop in with firepower. Banner, realizing that Fenris won't die with their ship, leaps from the Commodore and transforms into the Hulk, morphing when he lands on the Bifrost Bridge and taking Hela's beast down into the water. At the same time, when the fight seems lost, Loki arrives with the revolutionaries, landing on the bridge to provide support.
In the throne room, Hela gets the upper hand against Thor because of her immense strength and she manages to bring him to his knees, swinging with her sword and removing one of his eyes. As she takes him into her grasp, Thor has a vision of Odin's spirit, still in Norway, and he receives the insight he needs: Asgard could be anywhere. Even if Hela has taken it, then it's wherever the Asgardians are. Only Ragnarok can stop Hela.
With a massive bolt of lightning, Thor is empowered. He casts Hela away and uses his electricity to battle her army alongside 142 and Loki. Thor, knowing he has to stop Hela once and for all, asks his brother to help him destroy the city. While the Asgardians flee and he sends Loki into the vault to grab Surtur's Crown, Thor and 142 battle the Goddess of Death on the bridge and, sure enough, his plan works. Surtur is reborn in the Eternal Flame and Asgard is almost instantly destroyed. The prophecy of Ragnarok is fulfilled.
Thor saves his people from extinction while Hela perishes with Asgard. As they fly off into the cosmos, Thor thanks Loki for his efforts. There, in their spacecraft, Thor takes the throne before his people as the King and they head to Earth.