Krogg

A monstrous, mace-wielding behemoth from an unnamed extra-dimensional realm similar to the Phantom Zone, that existed between the dimensional barriers separating Earth-1 and Earth-2. Krogg was accidentally set loose upon Earth-1 by Superman of Earth-2 when he attempted to travel to Earth-1 to enlist the help of Jimmy Olsen to save the life of his friend, Jimmy Olsen of Earth-2. At the exact moment that he attempted to traverse the dimensions, Professor Potter was experimenting with the Justice League of America's Transmatter Machine aboard their satellite headquarters, and thus unleashed Krogg upon Earth-1.

Krogg battled Superman, and later his Earth-2 counterpart, defeating both easily. Using the Transmatter Machine, Professor Potter temporarily combined the two Supermen into one being to battle the interdimensional monster. This Super-Superman, as Professor Potter called him, was able to apparently destroy Krogg by increasing his internal energy, causing him to reach critical mass and explode.

(SF No. 186, Dec 1977: "Save My Friend, Kill Your World!"—SF No. 187, Feb 1978: "A Phoenix of Steel!")