"In ages long past, an incurable scourge ravaged mankind. A tiny menace that twisted men into monsters, the like of which you've seen. In Lucis lived a savior who could cure the afflicted. His body would come to host myriad daemons, that countless lives be spared. But a jealous King, one not yet chosen by the crystal, ostracized and demonized this healer of the people. Making a true monster of him. I gave you my name earlier but you should know it was not the name given to me at birth. Ardyn Lucis Caelum is my proper name. You'll never guess whose name Izunia was. Noct, killing you as a mortal will bring me scant satisfaction. Claim the crystal's power, arise as it's champion. Only once the crystal and King are no more...can I know redemption. Come back soon. I shall keep your friends company until you are ready." - Ardyn
"A man cursed with life eternal, whose immortality stems from the selfsame scourge that wrought the daemons. One so impure of body and soul was deemed unworthy of the Crystal's light, and forbidden to ascend. His mind twisted by spite and bent on revenge, the Usurper came to bring darkness down upon our world. Only the True King, chosen by the Crystal and guarded by his forebears, can end the Accursed's madness." - Bahamut
The crystal is a power source. It makes more sense the ones who generated this power pick and choose who can control it. Remember Bahamut resided in the crystal. And inside it is "Providence", a light capable of banishing the daemon infected darkness (Arydn). "Providence" becomes stronger over time as more "pure" kings take on the title King.
Ardyn was marginalized by the Astrals and denied ascension because with that many daemons inside of him the Astrals understood he was unclean/impure. He did not stop until he reached immortality and by then it was now out of spite. Other Kings and Oracles can purify without being corrupted, but Ardyn must have scummed to those daemons for the Astrals to leave him ostracized by his own people. Of course his word puts him in a sympathetic light. His actions... no. The Astrals denied him, not the crystal itself. Noctis, Luna, Regis, and I'm sure the original Ardyn weren't narcissist before and after purifying, which most likely was the current Ardyn's problem.
Both of the monologues you quoted refer to the Crystal as the one doing the choosing -- "one not yet chosen by the crystal," "only the True King, chosen by the Crystal," etc.
I don't think we're meant to see Bahamut residing inside the Crystal, either -- he appears there, but he also appears outside to fight Ifrit (and had done so previously to give his trident to the Oracle line), so there's no reason to believe it's his permanent residence.
Anyway, my understanding of the Ardyn situation is this:
* Ardyn saw the Starscourge wreaking havoc and decided that something needed to be done about it. He discovered that he could heal people by absorbing daemons into himself.
* Doing this made him impure. Therefore, he was deemed unworthy of the Crystal and was denied ascension. (Presumably in the sense that he either couldn't be a Lucii or couldn't be the Chosen.)
* Furthermore, he was demonized for this by another king who hadn't yet been chosen by the Crystal but eventually received Ardyn's last name as a title when he took Ardyn's place in the royal line.
My contention is that if the Crystal hadn't already been passed to the Lucis Caelum line, the Crystal wouldn't have bothered judging Ardyn in the first place, because the Astrals would have found someone else to fill that role in the first place instead of attempting to bestow the Crystal on him.
It's also implied that Ardyn should have been in the Cosmogony painting, which only really makes sense if he was a former King of Lucis after the Crystal had already become a known thing (since he wouldn't have needed to be there if he'd been cut out of the Crystal line before it even started).
As for the difference between what Luna did and what Ardyn tried to do, it seems like Luna's healing derived from the Astrals' light, while Ardyn attempted to fix the problem by taking the evil into himself. It makes perfect sense that he'd end up corrupted in the process and let his worst impulses take over.
The Six are the Astrals. The Astrals are the PHYSICAL manifestation of stars.
Astral = Star. "Astral" is derived from the Latin word Astra (star). Same thing. Sentient life forces originating from a star's power. They're that power in physical form. They created the Ring, gave a Tennebrae maiden the tradition, bestowed powers upon the Lucis and Tennebrae royalty, crafted the crystal, shaped the Starscourge, etc. There is no visible creator deity on disc and the links speak of none either. However, "the planet did it" comes up after magic/Astrals destroy bits of Eos. Suddenly Eos heals.
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Overall the Astrals are dicks who made Eos a living hell. Worst gods ever.
Eos itself is referred to as a Star, and the Crystal is referred to as its heart. The Ultimania claims that the planet produced the Crystal, which makes sense considering that. It's plausible that the planet is a higher power than even the Astrals.
As for the Astrals' competency as gods, it seems like most of the problems were caused by Ifrit's overt hatred of humanity, with the others generally only making things worse in the sense that they cause collateral damage when testing people. They're not omnipotent enough for their handling of chosen humans to seem like unnecessary cruelty any more than the behavior of the humans in question does. *shrugs*