Entities of an alternate dimension - the Demon World. Demons, unlike monsters are classified into two main groups: Underling and Overlord. Underlings are low to medium level demons (mostly beast-like demons). They lived as an Overlord's slave/minions/army but some prefer living independently in a society like the Imp. Overlord are high-levelled demons with great power and intelligence (mostly in humanoid appearance). They serve as Kings and Warlords, commanding both lower Overlords and Underlings.
Demons are also divided into 3 classes - Rampage, Shadow Walker and Necromancer. (The EA's classes were named after these demon classes because the classes were used to counter the three demon classes. This was originally used in C.A).
The most effective way of dealing with them is by using Exorcism but not everyone can do exorcism except the two exorcist clans that have existed during the demon war in the past, the Schade clan (exorcists) and the Kurozen clan (seal specialists). Some Demons are identified as ghouls, undead, and others.