Mark Horley was a Criminologist whose great intellect drove him to crime to satisfy his ego. In his lab, he created a formula which changed him in a Dr. Jekyll and Mr. Hyde fashion. By night he was a hideous Monster terrorizing the streets. By day he was working with the police investigating his own crimes. The Black Hood caught on to Mark's dual lifestyle and when confronted, the Monster took poison capsules and died.
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- Appearances of Mark Horley (Earth-MLJ)
- Character Gallery: Mark Horley (Earth-MLJ)
- Fan-Art Gallery: Mark Horley (Earth-MLJ)
- Quotations by Mark Horley (Earth-MLJ)
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