Thor, Vol. 1 Annual #7 (1978)

And Ever, The Eternals!
As Odin sends Hermod to Hel to see if there's anything to be done to save Balder, Thor begins wandering. He eventually finds himself in front of Mimir. Mimir tells Thor of a time in the past, that has been blocked from his memory. Thor left Asgard for Midgard, and during his travels, met the Eternals and learned of the Deviants and Mutates. Thor also learned about the Celestials, but Valkin blocked his memories until now.
Release Date: 1978