Arch Demons are the strongest of the Old One's servants, and the player must get through many lesser demons just to confront an Arch Demon. The player must kill at least one in order to pass through the fog gate beyond the Tower Knight Archstone. Once three are killed, the ambient music in the Nexus will change. All five must be slain in order to confront the Old One.
Demons comprise the majority of true boss battles in Demon's Souls. Their presence is indicated by a large health bar appearing at the bottom of the screen (provided HUD is on), ambient music particular to that boss playing, and an Archstone appearing upon their death. Most bosses appear once the player passes through a fog gate nearby. If the player defeats a boss in soul form, he/she will immediately have his/her body restored.