Little is known about them particularly to humans, though there have been reports of humans viewing them as gods or catastrophes. They are theorized to be akin to physical gods to demons. Most, if not all, demon species as well as humans are at least aware of their existence, regardless of if they actually venerate them or not.
They are said to be enormous and have power radiating from them with lesser and greater demons swarming around it. In this sense, the Monarch is almost like a queen bee.
All Monarchs are currently alive except the plant Monarch who was killed during the Great Human-Demon War.
Monarchs may be served under by Guardians, who protect them and enforce their will.
Guardians are demons who work directly under and protect Demon Monarchs from outside threats, particularly humans. It's been theorized that guardians exist for each and every monarch but that may not necessarily be the case for all of them. They typically are born into their positions and raised from birth as their soldiers and ambassadors.