Hi! Moo cow here.
Tauren priests, particularly shadow priests, are probably not as odd or as difficult as you may be presuming. The oddity really comes from tauren just seeming off, aesthetically, from a lot of cloth wearers. Being more connected with druids and shaman in game than anything else.
You can, of course, go with some of Yonara’s ideas here. Bought into the whispers of the Old Gods, a Twilight Cultist. Learned from other shadow priests about the shadow/void. Particularly Forsaken or Blood Elves, maybe some Sha stuff from the Pandaren, hexing and cursing things from Trolls, the opportunities there are endless.
If you wanted a more tauren route, you want to focus on Mu’sha, the moon. This takes a bit more of a astronomical and lore combo, but it works well. Mu’sha, the moon, is the left eye of the Earth Mother. Azeroth’s 2nd moon, Lo’sho, is her pain and sorrow yet uncorrupted. Mu’sha and Lo’sho remind the tauren of the Earth Mother’s ever watchful gaze, of her love and protection against corruption and the shadow. They do not shine as bright as An’she, however. They allow darkness and shadow and cover the earth and with it, cold and pain, despair and sorrow, anxiety and fear. Faith in the Earth Mother, in Mu’sha, keeps you safe in the darkness, that lone source of light.
A tauren Seer, who is a Shadow Priest, can tap into the darkness and use it against the foes of the tauren, or those who desecrate nature. Fill their minds with self doubt and fear, debilitating curses, the frigid cold without the warmth of the Earth Mother. What would come to pass without her protection and grace. To be teachers to the Tauren, why Mu’sha and An’she are so important. How one watches and gives hope, tricks the darkness into thinking it is safe, the other shines bright and beats it back.
Its only mildly different than a druids lunar abilities. The Seer uses the darkness the Earth Mother protects against as a warning. While the druid uses the power of Mu’sha for protection. A Disc Priest would be similar to a Balance Druid, philosophically. One must know both light and dark, solar and lunar, An’she and Mu’sha, that one cannot overpower the other.
Hope this helps in some way.