Dark Priests are a type of villains that are popular in many media, and despite this name, does not require the characters to be actual priests - any members of any clergy or cult is appliable for this position.
Who are they and What made them different with heroic/benevolent priests?
A "Dark Priest" is a priest/member of clergy/cult who uses his/her position to knowingly commit crimes that are deeply at odds with his/her religion, often they will try to convince themselves or others they are doing good but other times they may simply not care at all.
What makes them so dangerous is that despite his/her obvious evil, he/she can still hold great power in others and due to their status as a "man/woman of the cloth", he/she is often hard for a protagonist to counter without risking public backlash (especially if the setting is in the past, when the clergy had greater control than they do today). In many world religions, especially Christianity, a "Dark Priest" is also known as a false teacher or prophet.
Variations of Dark Priests
Corrupt Men of The Cloth
Men of the cloth serve a plethora of roles in all branches of media, both modern and ancient. They may be wise counselors, corrupt bureaucrats or confused and bumbling but generally well-intentioned old duffers. But every once in a while, a preacher crops up who is intimidating, charismatic and completely devoid of morality.
Some evil/deranged beings who infiltrate a society may also pose as a "Dark Priest" by adopting the guise of a cleric for whatever reasons they may have to worship demons or more simply to commit crimes.
Sinister MinistersThe Sinister Minister is a one-man Corrupt Church (and it is usually a man in this role, though female examples are not unheard of) in that all power typically ends up resting firmly in his hands and his hands alone as opposed to being shared out within an organization. He seldom employs Sidekicks or advisors, preferring to rely instead on his own usually very devious brain for scheming while entrusting the main grunt work to the hordes of unwashed and sanctimonious Mooks who make up the bulk of his followers.
Usually well-educated and cultured, the Sinister Minister is nonetheless unspeakably evil and regularly engages in acts of sexual depravity (or perhaps not; many a Sinister Minister is Straight Edge) and, if not outright violence, threats and intimidation. The Sinister Minister usually keeps his own hands relatively clean until late in the game when his power base is secure and he can act with impunity.