The jist of this expansion was Warrior of Light and Warrior of Darkness, like we heard in 3.4, is just an appellation with no real meaning—whether or not we're a WoL or WoD is strictly based on who's legend we're fulfilling (though I can't recall now if Hydaelyn herself uses the term WoL, but I wouldn't be surprised if She did so just because it's something we'd latch onto). Hydaelyn and Zodiark being Primals is probably true, though like Emet-Selch says, it's "after a fashion" I think. They were brought into being by the Ancient's "power of creation" which is the 'true art' with Primal summoning being a pale imitation of it—aether itself is created by the Ancients to aid in summoning, and we're told that (via sidequests in The Tempest, maybe? can't remember if it was MSQ) that our souls simply aren't strong enough to use the "art of creation."
Basically, sure, Hydaelyn and Zodiark are probably Primals, but on a scale never before seen summoned by a means that is now impossible. Do they still qualify as our run-of-the-mill banshin or toshin? The Ascians would have you believe that—and Emet claims Zodiark tempered them as a matter of summoning but... I'm worried about the how or why of that. None of their other creations seem to display this quirk, is it because when they imagine a God that's what they think a God would do? (Compel worship?)
The Ascians are the survivors of an apocalypse that took place on our planet some 12,000 years ago. They are probably the last surviving Ancients, well, Lahabrea Elidibus and Emet-Selch anyways. The others seem to be souls from the shards of that planet raised up unto Ascianhood somehow. (I think, anyways, I haven't pored over the dialogue enough yet to remember everything 100%.)
Basically, there was a super advanced civilization thousands of years ago with the power of what we'd consider Gods. (This is likely the mythical Age of Gods, and the Convocation of Thirteen being the basis for the Twelve, as always, wouldn't surprise me [Nald'thal is cited as 2 gods in Living on a Prayer in 1.23b, don't @ me]) For some reason, their wanton creation of new life beget spontaneous generation of new beings that were terrible and vicious—they considered these the manifestations of their darkest fears. These unintended summoning/creations made a lovely feedback loop of killing people that scared people into accidentally summoning more to kill more people to scare more people to summon more to kill people to scare people to summon more. This confuses me because the Ancients are immortal. We're also considered children by Emet-Selch's aetherial shades due to our small stature, which means the Ancients fucked reproduced. I personally don't understand how this all happens because we're only given information of how the planet works in terms of the Lifestream which may not have even existed pre-Hydaelyn.
The Ancients decide to give the star a will to fight off their misbegotten creations, and decide on summoning Zodiark. But this took massive sacrifices; something around probably ~75% of the population ends up getting used in powering up Zodiark. People get fed up with this sacrifice and summon an opposition who eventually win. I suspect, though I don't think it's stated, that the opposition saw the sacrifices to Zodiark as rather needless and "just as bad" as the Apocalypse eventually. They summon Hydaelyn, she wins, puts Zodiark in a moon and shatters reality (at least around the Source); through this life is forever saved in a cycle of reincarnation and life knows a paradoxical eternity and finality—something we're not sure the Ancients had. (This also dumped some Ancient souls into the Lifestream, of which it looks like the Player Character has some shard of one—probably the fourteenth member of the Convocation who left in opposition of summoning Zodiark).
Every Calamity has made mortal life stronger, especially in the Echo, according to Elidibus in 2.1, so it stands to reason it has something to do with the idea we're like 8/14ths whole now (the WoL being 9/14 after Ardbert).
As for: what is Light and what is Darkness? ¯\_(ツ)_/¯
Prior to Shadowbringers Light and Darkness were forces that did not interact with any other thing we've known of. Astral and Umbral were unrelated polarities of the six elements (the most common example is Astral Fire is the raging inferno of a flame, while Umbral Fire is the oppressive heat of a drought), and had nothing to do with Light/Dark which were reserved for Zodiark and Hydaelyn respectively. It was sort of similar to how void and Void are different, in that regard. The Void, the Thirteenth, was the Ascian's first bungled attempt at a rejoining, devoid of aether and home to monstrosities (that look remarkably similar to those that appeared in Amaurot at the Apocalypse). Now we've learned that Light and Darkness are also what the First call what the Source calls Astral and Umbral but in reverse.
Light is apparently stasis and Darkness is chaos; Astral was typically the 'active' and Umbral the 'passive.' It's just... messy now.
Lower case darkness, what the Dark Knights uses, (if memory serves) is instead fueled by emotion. Which, I guess is a certain sort of chaos, they impress upon channeling your inner darkness could be as simple as drawing upon the desire to protect those you love. Which, I suppose, could also be what the Ascians tell themselves too.
Is the Player tempered? Maybe, I guess. If we are, our directions while under the Tempering have been to "hear, feel, think" which is basically "live your life as if you were never Tempered" imo. Emet says people were tempered as a consequence of summoning Zodiark, but the player character wasn't the one who summoned Hydaelyn but just part of a person who did—which means killing all those Tempered people as the only 'cure' we did was utterly useless.
I've lost the plot here, but this story undid a lot of commonly held beliefs, and rewrote the rules Heavensward's Alexander set up for time travel; it's also casually 'retconned' some old 1.0 things. Like... the starshower everyone sees when awakening to the Echo is probably definitely supposed to be reminiscent of the Ancient Apocalypse and not the Battle of Silvertear now—though we knew the starshower visions also predated the Sixth Astral so.....