Eligos is a minor demonic entity summoned by the Necronomicon, which is a "Demon of the Mind". The demon is able to control and manipulate individual's minds, can inhabit the body of a mortal without displaying physical evidence of the possession, is able to teleport short distances, and has telekinetic abilities.
One such demon was summoned by Ash to find out how to undue what the Necronomicon had wrought. During the interrogation, the binding circle was accidentally broken, releasing the demon. It overpowered them, then unknown to the others it possessed Kelly. After which it manipulated its companions, fought them, and before it was exorcism killed Pablo's uncle.