The Rock of Eternity was the lair of the wizard Shazam. The Seven Deadly Sins were imprisoned here. It's main entrance to Earth is through Fawcett City. When it was destroyed it shattered into a billion pieces that littered the city. It was later reconstituted and now resides with multiple dimensions as to prevent such destruction from arising again.
Some time around 5,000 years ago, The Champion created the Rock of Eternity from two large rock formations — one from Heaven and one from Hell — to hold the "Three Faces of Evil", a dragon-like demon, captive.
Ibis, the Invincible
When Shazam felt disillusioned by the rising magical threats in the universe which constantly threatened humanity. He sought to understand human beings as he once had. Departing from the Rock, Shazam left it's power and responsibility to another capable wizard, "Ibis, the Invincible". Shazam, later, retook the position as he was reaffirmed in humanity and Ibis sought to reclaim the love of his wife.
The Rock of Eternity is a nexus of power in a nearby realm of existence that channels power to anyone utilizing the power of the wizard Shazam. It is a way station of power. When Shazam was removed from his position on the Rock it was left to Captain Marvel then calling himself just "Marvel".
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