A doomguard is a very rare magical golem made of steel plate armor and complex machinery powered by crystals. One of their great danger lies in their brute strength and near invulnerability due to the thick armor that makes up their body and protects the mechanisms within. They are also unaffected by most forms of magic.
The real reason they are universally feared, however, is the mechanism that causes them to split when physically injured. Although the doomguard can repeat this only a limited number of times, since the steel it is made of runs out, through some way of folding the doomguard holds enough material to duplicate itself many times over.
Adventurers have reported great success in swift, deadly attacks, forcing the golem to play to its weakness - its sluggish movements and encumberance - rather than its untiring endurance in drawn-out fights. Still, a person who can dispatch a doomguard, even with assistance, is surely someone to be reckoned with.