Masters of the Universe
In July 1982, Superman is patrolling the skies of Metropolis when he sees a "distinctly unearthly" cloud, a cloud which he is unable to penetrate with his X-ray vision. As he approaches it, he begins to feel very weak, and iss drawn into it by a mystical force.
He soon finds that the cloud is a gateway to the planet Eternia, and is a byproduct of an attack on Eternia's Castle Grayskull by the evil demonic Skeletor . (Castle Grayskull is a protectorate of the Masters of the Universe: He-Man, Teela, Man-at-Arms, a talking Tiger, and a Sorceress of divine origin.)
Superman, Skeletor, and his henchman Beastman engage in battle. Skeletor is impressed by Superman's amazing strength; nevertheless, the Man of Steel is no match for the mystical forces under the command of Skeletor, who throws Superman into the base of a nearby hill.
Soon therafter, He-Man arrives on the scene and immediately recognizes Superman, since his own mother is a traveling Earth-woman. No sooner are the two properly introduced than Skeletor casts an enchantment that effectively controls Superman's body and locks the two titans into battle. Superman is quickly victorious over He-Man, and Skeletor commands him to smash down the doors of Castle Grayskull...but before he can complete the terrible task, Superman is able to distract Skeletor with his heat vision and break free of his spell. He subdues Skeletor before he can cast another enchantment (DCCP No. 47, "From Eternia--with Death!").
In November of the same year, Superman again encounters the Masters of the Universe. While the space-traveler Zodac challenges He-Man after traveling through the Multiverse, Superman is sucked into an underwater vortex and deposited in Eternia. Superman's entry into Eternia is once again due to the meddling of Skeletor, who is seeking the other half of a power sword that lies on the bottom of one of Earth's oceans. Superman quickly grabs the power sword and hurls it before Skeletor can reach full power.
Meanwhile, Zodac is intentionally trying to lure He-Man away from a fight with Skeletor, in order to save his life. However, He-Man proves victorious and comes to Superman's aid. A battle ensues, and He-Man and Skeletor strike one another down, He-Man with Skeletor's own power sword, and Skeletor with a magical blast. Zodac arrives too late, assuming that He-Man is dead...however, he is merely knocked unconscious by the blast and otherwise unharmed.
At the battle's end, a dazed Superman stands up, wondering exactly why he is on Eternia in the first place (DCCP No. 51/2: "Fate Is the Killer").