I'm in the making characters based off of shadow-priests and shadow casters n' such from other RPGs. I've made a khajiit specializing in illusion and conjuration, who worships azura (the goddess of dusk and dawn) with the rationality that it is in between light and dark where the shadows lie, and I'm still trying to think of a back story that would promote why he became a shadowpriest.

So, recommended quests,mods, what I should do in general,etc...? Any help would be greatly appreciated

Requiem has "shadow magic" in the Illusion school. They're buffs that combine Invisibility+Muffle, Shield Buffs, Damage Magicka regen on enemies, things like that.

If you're going for that Shadow Magic, Familiar Faces could be used to replicate that whole "crossing into other worlds" thing like the Shadowkey multiplayer.

I had a character almost identical to yours, 'cept he doesn't worship Azura. His inspiration for shadow magic and magic in general came from a staff imported from Hammerfell that once belonged to Azra Nightwielder. Again, something similar could work if you're going for that Shadow Magic. For the other kind, maybe something with Nocturnal, too? She's heavily associated with Shadows, and Requiem bases its Shadow spells on her power in some way.

EDIT: I forgot to add, make sure you get the Soul Cairn summon spells. They fit the whole "shadow" theme well and since you're using Conjuration you may as well use them. My character didn't do Dawnguard, but I roleplayed it as him doing the research necessary to learn them himself as he was an accomplished Necromancer as well as a Summoner (the two kinda go hand in hand anyways, the Soul Cairn is on another planet after all).