The Hell Knight of Doom 64 are the ressurrected carcasses of those from Doom 1, resurrected by the Mother of all Demons. They are made from systematically altered dead carnage remade back into corrupted living tissue. The mutations were devastating. The Demons have returned stronger and more vicious than before.
Tough as a dump truck and nearly as big. These goliaths are the worst thing on two legs aince the Tyrannosaurus Rex.
One quirk of the game is that they can deal and it can receive missile damage to and from barons of hell, so infighting between the two monsters is possible.