Samuel Mackey

Samuel Mackey is  one of several incarnations of Satan who appeared in the X-Files, he acts as the primary antagonist of the episode "Signs and Wonders" - like most depictions of Satan on the show he is a powerful demon who disguises himself as a human in order to corrupt the world and this particular guise was based strongly on the Serpent aspect of Satan.

History
TBA

Abilities
Samuel Mackey is an incarnation of Satan and is thus an immortal entity with great demonic power, his particular realm of influence seemed to dominate over snakes and reptilian phenomena - he showed such malignant power as impregnating women with live snakes, summoning poisonous snakes to kill anyone he pleased and in the very end he even manifests his demonic form briefly by revealing a large snake living inside himself (though it is very possible this was demonic magic, since Satan is a spiritual being rather than physical in nature).