One of the things I love about the Warcraft cosmos is that there isn't one monolithic group or pair of groups behind everything. The Burning Legion is possibly the most dangerous thing in the cosmos, but they don't hold a monopoly on world-ending evil, as they have to share that with the forces of the Void - primarily the Old Gods - and to an extent the Undead Scourge (who may be a distant third on the power scales, but are still my favorite of the big three bads.)
It's really nice, because there can be real mysteries about the source of some threat or at least plot development. Contrast this with Blizzard's other major fantasy series, Diablo. In Diablo, if something's evil, it's almost certainly the work of one of the Seven Evils, usually Diablo. I really liked that Reaper of Souls inverted things by making it one of the Angels behind the bad stuff happening (with bonus points for demonstrating the difference between the theory of evil as a substance versus that of evil as a quality) but that still retained the idea that everything in the universe was either Demonic, Angelic, or a mix of both (though I'm given to understand the dragon Trag'oul might be an exception there.)
In Warcraft, the various factions of "pure evil" might interact with one another, but they are fundamentally distinct. But we are left with certain ambiguities on which to theorize. For example, we know that Yogg-Saron's influence on Vordrassil in Grizzly Hills is what created the seed for the Emerald Nightmare, but why, then, is Xavius so closely associated with it, given that he's a demon and thus should, theoretically, be totally opposed to the Old Gods and their works.
It's not just the bad guys who are divided into different groups, but Legion is beginning to explore some of the surprising connections between them. Let's do a spoiler jump before we get into that.
Elune has been one of the most confusing presences in the lore of Warcraft. She's the only classical celestial deity people seem to worship. The standard equivalent of Western Religion is the Holy Light, but while the Light is portrayed as benevolent in nature, it does not manifest as a single anthropomorphic deity within the cultures that worship it.
The Night Elves worship Elune. They deeply revere the Wild Gods (so maybe not a strict modern monotheism) but Elune is clearly paramount. For years, though, it's been unclear where she fits into the whole cosmos of the game.
She clearly must exist, because she is the mother of Cenarius, and we've met him in person. But she also seems clearly distinct from the Wild Gods like Cenarius' dad Malorne.
One popular theory for a while has been that Elune is actually E'lune, a Naaru.
Indeed, the influence of the Naaru, initially associated almost exclusively with the Draenei, now seems to play a role in all Light worship. Chronicle suggests that the first humans (or maybe Vrykul) to worship the Light learned about it through visions of crystalline beings - clearly Naaru.
But we learn in Legion (this is the major spoiler) that Elune is not a Naaru herself. She is, in fact, the creator of the Naaru.
That's a freaking huge deal. If nothing else, it implies that the Naaru did not spring forth from the Light as soon as the universe was created. But it also eliminates Elune's potential Naaru nature thanks to this relationship.
Therefore, it seems highly implied that Elune might be a Titan. The question is: what's going on with her?
First off: we know that the Titan Pantheon have all be killed. Some piece of them might remain in the Titan Watchers, but there's a heavy implication that they're really past reconstituting into their old celestial selves.
The exceptions here that we know about are Sargeras, who is heavily infused with Fel Energy, and the future Titan Azeroth, the World-Soul resting at the center of the planet where most of WoW takes place.
The Dark Trolls and later their Night Elf descendants worshipped Elune, who they believed slept within the Well of Eternity.
We know that the Well of Eternity was actually the wound left after Aman'thul extracted the Old God Y'Shaarj from Azeroth, and its waters were Titan blood.
So what are we to make of this?
I've seen one theory that suggests Elune is in fact Eonar, but Elune seems to still be quite active as a deity, whereas Eonar's last essence exists with Freya, who does not seem as powerful. (That said, the Keepers - including the four Keeper bosses in Ulduar - are actually very powerful. Freya is the one who created the Emerald Dream, or at the very least tapped into it and allowed Green Dragons and Druids to access it.)
Another theory that I find more convincing is that Elune is actually the Titan Azeroth. But that seems tricky too because we don't really have much evidence that Azeroth has really become a true Titan yet - that he/she might be more like a developing embryo.
It's also interesting that one of the Titanic Pillars of Creation is called the Tears of Elune, but it seems unlikely that Elune would have played a role in her own Ordering.
It's perplexing. We know that she created the Naaru, and it's also implied she's a Titan. But I don't know where she'd fit in the story. I'd watch this one carefully.