Lore
Even as they have been abandoned by their faith, so have the Forsaken abandoned the faiths of their living days. Very few still adhere to their old faiths, such as the Holy Light. Many no longer cling to any religion, placing their faith in their queen and their dark science. Most, however, have embraced new faiths of their own creation. The official religion of the Forsaken is the Forgotten Shadow, but some forsaken follow the Echo of Life or other philosophies, and some have even turned to the Burning Legion as a source of power, believing that only it is potent enough to defeat the Lich King.

The Cult of the Forgotten Shadow
The Cult of the Forgotten Shadow is largely based out of the church in Deathknell and the warrior quarter in Undercity. It is a religion that worships and uses the Shadow, and not the Holy Light. The members of the cult are evil and/or chaotic, and are taught to eradicate anything having to do with the Holy Light — and life in general. It is also a religion of divine humanism. In an early form of the religion, a Forsaken priest in Deathknell points out that the light and the shadow have both rejected the Forsaken, and power must be earned by themselves — according to a quest in World of Warcraft(however, they have since learned how to tap into the shadow). While they can no longer use the Holy Light and have since learned how to use the Shadow; the priests teach that there must be a balance between Light and Shadow, and members must learn the Light as well but to never forget they were born from the Shadow. However, it seems that they learn about the Light in order to better combat members who practice it, and defend against its undead damaging power. Being intelligent undead, the Forsaken fully understand the limitations and vulnerabilities associated with unlife. One of these disadvantages is that they can be turned, rebuked or even commanded by powerful positive or negative energy forces. Naturally, the Forsaken are always on the lookout for ways to limit or negate this vulnerability.

Echo of Life
Echo of Life is a belief system by some Forsaken that an echo of their former lives still exists in their undead state, and they try to gain access to it through the use of the arcane. Some Forsaken can't get over the fact that they are, for all intents and purposes, dead. Some accept the fact without issue. Both of these reactions can lead to arcane use, as the Forsaken seek to hear the echo of life. Forsaken who can't bear their undead condition feel more alive when wielding arcane power. The surge of power, so addictive to living creatures, proves just as intoxicating to the undead. Some scholars theorize that this addiction is purely psychological; others claim that a Forsaken's unique physiology makes them more vulnerable to arcane addiction. Whatever the truth, Forsaken who wield arcane magic can become corrupted just as quickly as living creatures. Forsaken who long for life love the feeling of vitality that the arcane magic grants. Forsaken who consider themselves dead and adjust to that fact might also wield arcane magic recklessly, believing themselves immune to magic's side effects. Such Forsaken hear the echo of life when channeling arcane power, but choose to disregard it.

Church of the Holy Light
Some forsaken have been able to learn how to use the light. Some wish to redeem themselves by putting their faith into the Light. Needless to say these Forsaken are often mocked and shunned by others of their kind. Forsaken priests of the Holy Light are extremely rare, and would probably cut their bonds with the Forsaken race.

Roleplay
All forsaken can choose to follow any of the above paths, but it will have the most inpact on Priest characters. Unless you decide to play one of the very rare Holy Light Forsaken priests you will -not- be able to use the Light, your spells come from elsewhere. You don't believe in a god-like being, more in a way of life. The forsaken priesthood is not extremely orginized, leaving its members to further their goals as they see fit. Needless to say, Forsaken priests are in general not the benevolent healers and spiritual guides that most priests of other races are.
Although priest would be the first choice to play a Forsaken IC priest, both Mages and Warlocks could be adapted to serve as an IC priest.

Most common IC Classes a Forsaken Priest could be: Shadow Ascendant, Apothecary and Necromancer
See Common Forsaken Classes for more info.

Source: wowwiki.com

Very nice piece Xon, thanks for posting it.