It seems to be that the Death Mask was originally an unenchanted mask owned by a great and powerful wielder of necromantic energy-either a reaper or a necromancer. It was just an ordinary mask but then was enfused with death energy and then given to Peter Eldon when he was reanimated into a death guardian towards the end of the Bone War. The necromancer who reanimated Eldon was given it as a gift and he carried it, in the hope of making one of his death guardians stronger. When Peter killed this necromancer he found the mask in a bag of loot and the mask proved to be a powerful item. Baron Deathmask would then go on to be known for this iconic mask.
The mask has been described by several accounts, though most of them are unclear. The fact is that most people who ever do see the Baron's mask end up dying in combat with the death guardian. Only a small percentage have survived to tell the tale.
The mask is described by most accounts as being white with no symbols or art on it, with a thick green glow of death energy surrounding it. It is very much like that of a Guy Falwkes mask, but without the details. The second type of account is where some people have said that the mask is black, revealing that it is very dark. As shown on the photo on Baron Deathmask's page, this might be connected to a helmet or the helmet itself maybe the mask. It is uncertain, but it is most likely the mask is white.