The quest "Bonds Forged Through Battle" delineates the path to the formation of the Lightbound. The Draenei started by more peaceful sermonizing, to which some Orcs responded willingly. As their zealotry increased they started forcibly converting people and turning them against those who didn't respond to their entreaties. The Shadowmoon Darkcasters are kind of the marquee Priest template for Mag'har Priest NPC's, but this quest still gives a canon rationale for Holy and Discipline Priests among the Orcs. Not to mention that the Shadowmoon Clan had a strong proscription against using the power of the Dark Star (K'ara), precisely because of its dangerous and deleterious effects. The acts of AU Ner'zhul were not accepted by the more conventional Shadowmoon, represented by Rulkan, who considered the use of the Void and necromancy as a horrific defilement of their ancestors. With the defeat of Ner'zhul and the breaking of the Iron Horde, I would imagine most Shadowmoon share in that traditional proscription, and follow Rulkan's views as opposed to those of Ner'zhul - they wouldn't really be into the Void, despite their use of Shadow magic.
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Apparently they're in the Lightbound forces: