But I mean the lack of some important Shadow Priest characters in World of Warcraft, some Gul'dan or you know?
If you check out Warlock or Death Knight lore, you find a lot about Arthas and the Lich King and Shadow of Council, Gul'dan and the Burning Legion, it's just quite many connections.
I just wish perhaps there was moe Shadow Priest connections or important characters that was used in previous expansions or future.
I hear ya, I wish there was more Shadow Priest lore too. I know the just of it: "Forsaken priests found shadow magic and taught others in secret and it spread and now every priest knows that light cannot exist without shadow" or something but that's pretty much it.
If you search for priest lore you have to look up paladins and warlocks and sometimes even shamans a lot of it is hidden there.
But you are right. There is basically shadow lore for the undead and holy lore for the others, but those are just dublicatees of each other and what is called shadow lore for the undeads is basically an explanation how they can use the effects that are realised ingame as holy spells. It is not lore to explain the shadow tree itself.
What we need is new lore for non-Undead shadow priests. Benedictus and Anduin are official priest characters with shadow powers, yet we don't know how they learned it or... even why, both characters have the Light on their side. We need a dark priest character to represent us!
There never was any indication that the shadow tree for non-undead priests is not a side effect of learning to use the Holy ight itself, kind of like you studying medicine will not only help you to understand how to heal people but also how to harm them.
Since the holy energy priets use is mostly not produced by them but also just transferred I always thought of the shadow school spells as a forcefull taking of the holy energies from the target.
It fits well with the explanation for how holy magic, shadow magic and undeads work together (also, the first blood elf paladins) to see both schools as manipulating the same type of energy which comes in positive and negative from, both oppose each other and upsetting the balance causes harm.
It explains how they can cause almost the same effects but with a certain twist.
Even abilities like atonement and vampiric embrance fit in very well.
Atonement forces holy energies through your enemies causing them harm before spreading out and healing your allies, vampiric embrance takes holy energies from your enemies channeling them trough yourself and using them to heal your allies. Vampiric embrance on undead is kind of a problem though and you would exspect atonement to harm undead more and heal less.
If there was ever an expansion where Shadow Priest could shine in new characters or what not, it would been Mists of Pandaria, now that's over. Cus of the whole darkness within, manifistations, Sha appear, light\darkness(shadow), yeah.
But we can hope for the Prince of Stormwind to turn Shadowly-evil!
Anduin would be content with Holy Fire / Smite for the time being. Only way I could see him going Shadow would be if he became King and was fighting a very tough battle.
Sadly most Priest NPCs who do battle as Priests do so without the aid of Shadowform even when using Shadow damage spells like Mind Sear, Mind Flay, Mind Blast or Mind Spike. It is sad to see the lack of Shadowform outside of players in the game especially when you consider the level of ability and skill that it takes lore-wise to achieve Shadowform.