Demons are evil or unclean spirits (cf. Mk 1:23 with Mk 1:32-34; Rev 16:13-16), and are fallen angels, servants of Satan (Mt 12:26-27; 25:41). There is only one devil, but myriads of demons who serve the devil and make his power practically universal. A demoniac (Mk 5:1-20) is a person whose personality has been invaded by one or more demons, who at will can speak and act through their human victim, deranging both his mind and body.
A number of such victims of Satan were delivered by the Servant (see Expulsion of Demons below). The unhindered power of God working through the sinless humanity of the Servant challenged the supernatural world of evil and explains the outburst of demonism during His earthly ministry.
The reality and personality of demons are attested in all eras of history since the Fall, as in the case of Saul and the spiritistic medium of Endor (1 Sam 28:7-20), in the case of ancient idolatry of which demonism was the dynamic (Ps 106:36-37; 1 Cor 10:20), in ancient divination and magic, and in ancient necromancy and modern spiritism.
Demons can derange mind and body (Mt 12:22; 17:15-18; Lk 13:16). They know the deity and lordship of Christ in the spirit world (Mt 8:31-32; Mk 1:24; Acts 19:15; Jas 2:19), and realize their predestined fate (Mt 8:31-32; Lk 8:31). They have a conspicuous role in the government of the Satanic world system (Dan 10:13; Eph 6:12), in promoting cultism and false doctrine (1 Tim 4:1-3), and in opposing Gods program and Gods people (Eph 6:12; 1 Jn 4:1-6). Prayer is the believers resource against Satan and demons (Eph 6:10-20).