Crown of Surtur

"Smaller than I thought."

- Hela

The Crown of Surtur is the headpiece of the Fire Demon Surtur, when merged with the Eternal Flame, was capable of resurrecting Surtur and increasing his power immensely.

Escape from Muspelheim
Two years after the Battle of Sokovia, Thor had been unsuccessfully searching for the Infinity Stones, and was now imprisoned by the fire demon Surtur. Surtur boasts that Asgard will soon be destroyed in the prophesied Ragnarök, once Surtur unites his crown with the Eternal Flame that burns beneath the city. Thor kills Surtur and claimed his crown, believing he had prevented Ragnarök.

Ragnarök
"By the Eternal Flame, you are reborn!"

- Loki

Some time later when Thor came to reclaim Asgard from the clutches of Hela, he found it too difficult a task to defeat her alone. With the surviving population of Asgardians disembarking from Asgard on a large lifeboat, Thor made the decision to actualize Ragnarök and use Surtur's power to defeat and kill Hela. Knowing the Asgardian people were out of harm's way, he ordered Loki to place Surtur's crown in the Eternal Flame, thereby resurrecting him. Surtur erupted from Asgard moments later, laying waste to the helpless city before him. Surtur thrust his blade, as foretold in the prophecy, straight through Asgard, which incidentally killed Hela along with himself in the process.