In the Warcraft universe, there are some clear categories of beings, even aligned with various primal forces.
In fact, there's even a chart in Chronicle that explains it:
Demons are the embodiment of chaos and use Fel magic.
Undead (with the Lich King probably as the ultimate manifestation) are the embodiment of death and use Necromancy.
The Void Lords (and to a lesser extent the Old Gods) are the embodiment of Shadow/Void and use... Shadow/Void magic.
Titans are the embodiment of order and use Arcane magic.
The Wild Gods (Ancients, Loas, and Celestials) are the embodiment of Life and use Nature magic.
And the Naaru are the embodiment of the Light and use Holy Magic.
But the Night Elves worship a Goddess named Elune who doesn't fit into any of these categories.
In Legion, we've been getting closer glimpses as to Elune's nature than ever before, and they are not exactly clearing things up.
First off, the five artifacts we want to use to lock up the portal in the Tomb of Sargeras are all named after Titans - the Tidestone of Golganneth, the Hammer of Khaz'goroth, the Aegis of Aggramar, the... Eyes(?) of Aman'thul (it's the one in the Nighthold raid, so we haven't seen it yet.) And then there's the Tears of Elune.
So is Elune a titan?
Well, she was not part of the Pantheon, as far as we know. The Pantheon were the above listed as well as Eonar, Norgannon, and of course Sargeras.
Is it possible we've been working with an incomplete list?
Unlike the other non-Sargeras titans, Elune has made her presence known (you know, since she's still alive,) most recently purifying the Tears of Elune and rescuing Ysera's spirit from the Nightmare (um... spoilers. But it's almost been a month.)
Could Elune be a Titan who was not part of the Pantheon?
Let's come back to that.
The other thing we find out kind of off-the-cuff from Xe'ra is that Elune created the Naaru, or at least the first Naaru. Given the longstanding speculation that Elune was a Naaru, this kind of fits with that even as it disproves it.
Now, there's nothing preventing a Titan from being involved in the Holy Light. Tyr and Odyn both clearly go in for the whole Holy thing (even though Odyn's kind of terrifying - he's a little too into that whole "glorious combat" thing.) But Xe'ra claims that Elune created them at the very beginning of the cosmos.
The Titans are for all intents and purposes gods, but they did not arise at the beginning of time - they awakened gradually over many eons, each starting off like the World-Soul that resides within Azeroth.
We're told that Aman'thul was the first of them to awaken, but perhaps that only means he was the first of the Pantheon, not the first Titan over all.
Elune is associated or even equated with the moon - actually, we should specify the big white moon, as Azeroth has two of them (and there's almost no lore associated with it, unless that's what Worgen are referring to when they say "May the light of the New Moon guide you," because if they mean New Moon like we do on Earth, that's basically saying "May that light that doesn't exist guide you.")
Given that Titans are born of heavenly bodies, could it be that Elune literally is that moon? Perhaps she is a kind of minor Titan - not a planet, but a moon.
Or, there's my tin-foil hat theory.
Elune is sometimes referred to as Mother Moon. Makes sense - Night Elf society is at least partially matriarchal, and so it could be just a metaphor.
But let's suppose instead that Elune is not just really a Titan, but a fully-fledged one, and even one that pre-dated Aman'thul.
And the kicker: Azeroth is her daughter.
Azeroth has the potential to be the most powerful Titan ever to exist - that's what makes the stakes so high (as if the mere fate of the world that all our characters lived on wasn't enough.) It's why the Old Gods are so determined to corrupt her and also, probably, why Sargeras is so intent on destroying her (as he previously was the most powerful Titan, something he proved by killing the rest of the Pantheon.)
Why is it that she is so special?
Well, what if Elune gave up most of her power - poured it into this other heavenly body - in the interest of creating a child that was greater than she was. The Titans don't, as far as we know, reproduce in any conventional way. But maybe Elune was pioneering this idea.
Her own power diminished as a result, and thus when she had finished, she was reduced in size and power to this lunar entity, rather than a full planet.
This explains why Elune is in this separate category from the other Titans, as well as explaining why Azeroth is so special.