Final Days
The Final Days refers to a cataclysmic event that almost brought about the end of Etheirys, once in the time before The Sundering, and once in the Seventh Astral Era. The cause of the Final Days was originally unknown, with the Amaurotine people hearing a strange sound that seemed to echo out from the core of the planet, followed by their Creation magicks running out of control and producing horrific monsters later known as Blasphemies that began to destroy the star. In order to prevent the end of their world, the Ancients called upon Zodiark, the first Primal, to protect them, albeit at the cost of a great many lives.
Later, when the Final Days returned to Eorzea, due to the Sundering thinning the people's Aether, they experienced transformations into the Blasphemies, with their Aether being consumed in the process. When the Warrior of Light defeated Zodiark, the shield of Aether that surrounded the world was destroyed, allowing for the return of the apocalypse. As it turned out, the cause for both instances of the Final Days was Dynamis, the lesser but more volatile source of energy that made up all of existence. During the time of the Ancients, the people were made of incredibly dense Aether, and were therefore unaffected by the Dynamis. However, their creations were susceptible to the way powerful emotions interacted with Dynamis, which caused their creations to become warped, which in turn inflicted fear and panic on the people, which continued to cause their powers to be affected. In the Seventh Astral Era, Dynamis was able to more directly affect the people, which fed on their Aether to transform them into twisted monsters, spreading dread and fear as they had before. When the people were able to master their emotions, they stopped transforming.
The Dynamis was also being affected by the Meteia, creations of the ancient Hermes, who sought to learn of other worlds. When she and her sisters became overwhelmed with despair at the death and destruction of other worlds, their hivemind shared this misery with the others. In time, Metion's control over Dynamis became its chosen method to destroy Etheirys, in order to spare its people from such a fate. To achieve this goal, Meteion left for the edges of the cosmos, far from any being of Aether, and channeled the abundant Dynamis at the planet.
When the cause of the Final Days was discovered by the Warrior of Light visiting Elpis and meeting Emet-Selch, Hythlodaeus and Venat, who would later become Hydaelyn, and learning of the events that led to Meteion fleeing to Ultima Thule, the Warrior of Light confronted her at her nest and defeated her as The Endsinger, stopping the flood of Dynamis to the planet.
Lore
“This dreadful calamity befell the star in the distant past, when it was known as Etheirys.
In that shining age, men had the power to manifest anything at will, even living beings, and through this miraculous creation magick their civilization thrived. One day, however, following a keening sound from the land, their magicks ran rampant, giving form to eldritch horrors born of fear and despair. Their blissful lives became a waking nightmare, and mankind was driven to the brink of extinction.
The Convocation of Fourteen, the people's chosen representatives, observed that the phenomenon manifested where the celestial currents, the star's outermost flow of aether, had grown weak and stagnant. In order to address this deficiency, they summoned forth the mighty Zodiark to exert control over the laws of creation. Though this act demanded a high price─the sacrifice of half of mankind's remaining number─the vast majority willingly rendered up their lives, and thus were the Final Days forestalled.
Over the intervening millennia, this star-encompassing calamity had faded from memory. But faced with its second coming as engineered by the Telophoroi, the Scions of the Seventh Dawn embarked upon a desperate quest for the truth. At length, they discovered that an ancient named Hermes had created Meteion, a being capable of traversing the great expanse, to ascertain the meaning of life by seeking out other civilizations in the universe. Alas, Meteion and her sisters found naught but despair, driving them to conclude life was suffering and death the sole mercy. Fleeing to the edge of existence, they became the Endsinger, and proceeded to unleash the power of dynamis, an esoteric energy set in motion by feelings, to usher all stars to their doom, that life need not continue in vain.
Despite seemingly insurmountable odds, the Scions held fast to hope, and amidst darkest despair lit the way for their champion as they confronted the Endsinger at her nest. And at the end of a fierce battle, they emerged triumphant, and put an end to the Final Days once and for all.