Unlike the Holy Priest, which is a standard of MMORPGs and indeed fantasy in general, the Shadow Priest is a bit more of an unusual beast; while dark priests can certainly be found throughout fantasy, they’re generally evil and nearly always on the side of the baddies. But this doesn’t seem to be the case in World of Warcraft; there’s no stigma attached to Shadow Priests as there is for warlocks or even rogues. You can get training in the Shadow in the Cathedral of Light at Stormwind, if you so please. Clearly the idea of “good” holy priests and “evil” shadow priests is flawed, even before you get into the hoary old notion that everyone is a unique and special snowflake, unable to be pigeonholed into convenient labels.
This last notion is absolutely untrue, and I point to the Warhammer Online trailer as proof – Pretty people wearing lots of clothes are good, ugly creatures or those wearing not much clothing are evil. See? It’s easy. And so true, too.
The Stereotype: An avatar of some dark power, who revels in the malefic puissance he receives. Usually younger than your stereotypical Holy Priest, the Shadow priest has at some point decided that service and sacrifice are for chumps, and that the highest cause should be oneself. (Some better read players may well work a bit of Anton LaVey to the mix.) They’re patient in a leering, I-can-destroy-you-whenever-I-please kind of way, and while they may be just as wise as a holy priest, their wisdom runs less to “Do unto others as you would have them do unto you,” and more to “Vanity of vanities! All is vanity!“. Essentially, an inversion of holiness, a mix between Nietszche and Darth Vader in cloth armor.
Alternatives: The Judge (Shadow Version): A true believer in a very good deity (such as Elune or the Holy Light), the Judge knows that the works of god must be kept righteous, and that sometimes the only creature who can find the rats in the wheat-field is a snake. They have devoted themselves to the Shadow not for themselves, but to find those who lurk in them. While you visit your god’s wrath upon those unbelievers who would threaten it, your true targets are not outsiders but the corrupt and heretical; those foul and selfish priests who would make their religion serve themselves rather than others. The city priest who drains his parish’s wallets with urgent prayers for tithings and then helps himself to the collection tray, or the town minister who receives the most vulnerable people in his care and preys upon them for his own pleasure; these are the ones who will meet you once on a moonless night. Part Angel of Death, part CIB detective; you answer not to the church but to your god alone. After all, whom among the church is invulnerable to corruption? You can answer to none of them. All must be accountable to god. And to you. (Definitely fits the Alliance races better, particular the Night Elves, Dwarves and Humans. This concept requires a church with a decent hierarchy to investigate, and the Horde races have a much different model of priesthood.)
The Exile: You didn’t leave the church. The church left you. Once upon a time, you were holy, and true. You believed. But then something happened, a trauma that left you scarred and realised the church had nothing to offer you. Either you never talk about what trauma it was, or else you talk about it endlessly and bitterly. But once you left, you bitterly embraced its opposite. After all, you reasoned. A guy’s gotta survive. You can’t rely on anyone else. This would have made a good story. It would have. But there’s one fly in the ointment: You still believe. Deep down, you know you’ve gone wrong, you know you’ve taken that wrong path. By day you drink and curse (in more ways than one), but you cry yourself to sleep at night begging for forgiveness. You do not revel in your dark power. It is your cross to bear. Maybe, somehow, you’ll find a way to earn redemption. (Conveniently, the game features a way to mechanically represent that redemption: Respecs. Race wise, the alliance races all make sense for it, but the Blood Elves en masse could easily fit this archetype; they have rather a massive trauma in their past to face down, after all!)
The Dark Counsellor: Quiet. Ever, ever quiet. That’s the word used so many times to describe you. But you’re a loyal part of the group, you say your prayers and wear white robes. You are trusted as wise, and many seek your “holy” advice. This suits you tremendously. Much like your fellow shadow priests, you envisage a future guided by darkness and destruction, you see yourself as an avatar of the shadow. But, unlike your foolish brethren, you know the odds. You’ve worked out that on your own, you cannot triumph through force and shadowy might. But a poisoned word here, and a warrior defends, because you should not judge, a man who he should be destroying. A sinister whisper there, and the mage is isolated within her research, her capacity for good trapped in an Ivory Tower. You distribute good intentions to all around you, and let them pave the road themselves. (This is an excellent choice for a Troll; desperately trying to preserve the ways of voodoo with trickery and deceit in the orcish halls of leadership. Pretty much any race, however, could make it work. The idea of a corrupt counsellor is as old as time itself.)