Per an E.A.G.L.E. file from the 1970s:
Whether the subject is in fact demonically enhanced, or whether more conventional explanations exist for his paranormal abilities is at present unknown. What is known is this: While "in-country" north of the Mekong Delta, subject's entire Marine patrol was captured by North Vietnamese forces.
Was a Marine Master Sgt serving in the American segment of the Vietnam War. Black magic turned him into a monster. He held a vendetta against Braintrust in the mid-70s, purposely trying to wreck his organization more than other gang leaders possibly over a girl.
When the Dark Age peaked, he morphed into something even larger and more monstrous, as Black Velvet's energy radiated the city in January 1977 when he was one of the heroes trying to help protect the city. The combined might of the heroes and vigilantes of the time(though Honor Guard was out of town) could barely contain the beast that he became. He eventually returned to Vietnam for a few years before returning.
He is bullet proof and has super strength.
In truth, Hellhound's abilities in fact stem from nothing the least bit conventional. When Hellhound loses control of whatever magical force is held in his body, he transforms into a giant out-of-control kirin. He was taught to hold the entity within him at bay by someone he refers to as the "old man."
In later years, the nature of this bond changed. Perhaps it worsened, perhaps it simply changed, but in his days as an Omega Ranger, Hellhound had come to take on more of the kirin's physical features, becoming more pronouncedly demonic in appearance and even sporting a tail.