Black Zero is a member of a ruthless empire of pirates and terrorists that specialize in the destruction of planets. He is described as having no fingerprints, purple eyes, and a forked tongue.
In April, 1968, Black Zero sets his sights on Earth and plans to obliterate it with an anti-matter missile. His plans are foiled after Jimmy Olsen offers a "Life-Saver" candy to Clark Kent, thus inspiring Superman to create a hole through the center of the Earth so that the missile can pass through it without touching it. Black Zero is ultimately destroyed by Jax-Ur, whom Superman has temporarily released from the Phantom Zone. Before returning Jax-Ur to the Zone, Superman credits Jax-Ur as "the man who avenged Krypton's doom!" and the two plan to team-up again in the future when they hope to "get a crack at the men who ordered Krypton's destruction -- the rulers of the PIRATE EMPIRE!" (S No. 205, Apr 1968: "The Man Who Destroyed Krypton!").
During his time on Earth, Black Zero reveals to Superman that he is responsible for the explosion of Krypton. This fact is not confirmed elsewhere in the chronicles and is often contradicted.