The Arc Demon (or Arch Demon) is a demon who uses the powers of darkness to wreak havoc upon the world. He is the penultimate member of the classic demons' family, generally ranked above a Devil but below a Demon Lord. They are essentially a stronger version of the Devil.
In some occult and similar writings, archdemons are spiritual entities, prominent in the infernal hierarchy as the leaders of the demonic hosts, just as archangels lead choirs of angels. Essentially, archdemons are the evil counterparts of the archangels and are possibly referred to as "archangels of death" in the Bible.
Archdemons are occasionally described as once being rebellious angels who had fallen from Heaven and which maintained their ranks as ex-angels within their new roles. Alphonsus de Spina in 1459 believed that specifically one third of the original angelic court became devils. However, accounts have varied throughout history and Johann Weyer altered the figures to 7,405,926 demons and 72 princes of Hell.
Some examples of archdemons over time include Belial, Ashtaroth, Asmodeus and Lucifer.
In Circle of the Moon, Arch Demons appear as green goat-like beast men that spit out blasts of Darkness magic in rapid succession. They (and all demons, for that matter) can be easily dispatched if the player crouches and attacks right in front of them, making their barrage of fireballs miss.
In Aria of Sorrow, Arc Demons are no longer immobile and instead hover around the room to launch large fireballs toward Soma. Their appearance is no longer goat-like, instead they bare a more humanoid shape. They possess a large pair of claws, two pairs of wings and a pair of elongated horns. The player should be careful not to stay near them, lest the demon responds with a quick claw swipe.
In Dawn of Sorrow, Arc Demons create magical seals that unleash three fireballs that home in on Soma. They also try to kick him as they fly around. In this game they are green colored, have a torn pair of wings and have two pairs of horns, one on the side and one coming from the top of their heads. They only appear in the Abyss and are notorious for having a lot of HP (even for high leveled characters wielding advanced weaponry).