The Eternal Flame was a mystical, inextinguishable flame that contained the true power of the Fire Giant Surtur, and is capable of resurrecting deceased individuals.
During the confrontation between Thor and Surtur, the latter explained that Ragnarök would occur when his crown would be merged with the Eternal Flame. Later, Hela resurrected her army of Berserkers and Fenris by using a handful of the Eternal Flame. As Thor and his Revengers were fighting Hela's army in Asgard, Thor understood that in order to defeat her, Asgard, Hela's power source, must be destroyed. Loki dropped the Crown of Surtur in the Eternal Flame, resurrecting Surtur and unleashing Ragnarök.
The Eternal Flame contains the full power of Surtur that was taken from him when Odin defeated him. By placing the Eternal Flame into the Crown of Surtur, Surtur can be summoned and even resurrected should he already be killed and restored to full power, allowing him to unleash Ragnarök.
The Eternal Flame also has the power to resurrect deceased individuals, as shown when Hela used a handful to resurrect her army.
It's power was so great that it was the only thing Hela openly praised for in Odin's Vault, with even the Space Stone being dismissed in favor of the Eternal Flame.
- In the comics, the Eternal Flame is a mystical flame that cannot be extinguished. It was stolen from Surtur by Odin, Vili, and Ve Borson at the dawn of time so that Surtur could not light his sword and bring about Ragnarök.