Thor (1962 1st Series Previously Journey Into Mystery) #425

Hercules, Eric, and Thor return to Asgard after being warned by Sif; They find Surtur and Ymir locked in combat; To defeat them, Thor must retrieve Twilight from the Sea of Eternal Night; During the battle, Thor becomes trapped, and Eric makes a fateful sacrifice; He bridges Thor and Mjolnir, and is re-merged with Thor; Thor lights Twilight in the Eternal Flame which causes Odins control over Surtur to vanish; Surtur and Ymir fight over Twilight, and while they are distracted, Thor transports them both into the Sea of Eternal Night, where they will be frozen for all time.