Beast of the Final Days
Beasts of the Final Days are beings once composed of aether, but were consumed by dynamis and twisted into creatures devoid of reason. Their appearance on Etheirys signals the coming Final Days, the destruction of the star.
The Final Days were twice accompanied by the appearance of a terrifying menagerie of twisted beasts, which laid waste to the resplendent world of the ancients and caused untold suffering in our own. Yet despite the indelible strain they have left on the pages of history, much of their true nature is not widely understood.
— Encyclopædia Eorzea Volume 3, p. 282
Ancient Era
Long ago, during the time of the Ancients, the people of Etheirys were able to form the aether around them into anything they wished through their Creation magicks. Being made of incredibly dense aether, they were unable to interact with the mysterious force known as dynamis, however their creations were not as dense. When Hermes created Meteion in order to more closely study dynamis, he also created replicas of her to send to the various planets beyond their star to learn the meaning of life. However, upon discovering nothing but death and dying planets and people, Meteion became overwhelmed by the despair her sisters felt, and determined that the meaning of life is suffering and pain. She determined that Etheirys must die as well, in order to avoid such suffering and pain. Due to being a creation, Meteion's aether was thin enough for her to gain control over dynamis, and she declared that she would bring about the Final Days.
Following the events at the Ktisis Hyperboreia, Meteion fled to the edge of the cosmos and began to "sing her song of oblivion", channeling dynamis into the planet. The sound of her song rang out to the planet, a strange sound that began affecting the creation magicks of the Ancients, twisting them into fierce, unthinking monsters. As panic spread through the star, the creation magicks began to run amok, birthing more and more of these monsters that slowly began to consume the world. This was only stopped by the Convocation of Fourteen by calling upon Zodiark and shrouding the world with aether. When Zodiark was sealed away by Hydaelyn and the world was sundered, the aether remained around The Source, protecting it and the much more fragile inhabitants who called it home.
As the shroud of the Final Days descended upon the ancients, out-of-control creation magicks gave birth to the beasts that would spell the end of their golden age. While the conjuration of life from aether was commonplace among the people at the time, the process had ever been bound by a rigorous set of rules. This chaotic period saw each of these rules shattered in turn, with the unpredictable appearance of monstrous entities wreaking untold havoc—and, for the first time, gave the ancients reason to fear the power they possessed.
The cause lay in Ultima Thule, at the very edge of existence, whence the Endsinger sent a wave of despair to sweep across the sea of stars. Once Etheirys was caught in the churning tides of dynamis, men began to witness their darkest fears becoming reality. For one poor soul, this meant meteors raining destruction upon the land; for another, it was a city burned to ash by unquenchable flames. Yet another saw an abomination from the realm of nightmares come to life—the First Beast of the Final Days.
— Encyclopædia Eorzea Volume 3, p. 282
Modern Era
In the modern era, the people of Etheirys are made of much thinner aether, and are therefore far more susceptible to the effects of dynamis. When the barrier tied to Zodiark is destroyed, dynamis is able to manifest and feed on the aether and negative emotions of the people, destroying the aether that they are made up of if they succumb to their despair. Due to being affected by emotions, the beasts can be driven by a single-minded goal or wish the person once held. However, in most cases, the beast become violent beings devoid of reason. In the extremely rare occasion, a beast can become intelligent, capable of strategizing and commanding the other beasts.
The primary difference between the first and second comings of the Final Days is the manner in which their calamitous beasts were made manifest. While the monstrosities of eld were, as previously mentioned, created by an outburst of creation magicks, those of the present day were the result of innocents being transformed by the swells of despair.
This variance arises from the fundamental nature of the ancients versus those alive today. We are composed of much less dense aether, thus our inability to wield creation magicks. This paucity of aether, however, makes it easier for us to manipulate dynamis, a force driven primarily by emotions. The physical metamorphoses observed during the Final Days arose from such sudden changes to the dynamis—birthing beasts shaped by the dark hands of despair.
Similar phenomena have taken place in which aether, the constituent energy of body and soul, is warped to create creatures such as sin eaters and voidsent. However, since beasts are wrought from emotion unbound by physical limitations, dynamis gives rise to more extreme results. Furthermore, when one succumbs to transformation into a beast, naught remains in their heart save the singular sentiment that inspired their despair. This is thought to be the reason behind beasts' single-minded behavior.
— Encyclopædia Eorzea Volume 3, p. 284
Blasphemies
Blasphemies are particularly strong beasts of the Final Days that are always the first to emerge in a given geographical region, due to originating from people with the greatest levels of despair. The hunting of blasphemies is the focus of the Endwalker Role Quests.
Witnessed in Radz-at-Han, the initial beast to emerge from the chaos of the Final Days was branded "blasphemy" for its twisted resemblance to Thavnair's divinities. Leaders from Eorzea and the Far East followed the lead of their Thavnairian allies, adopting a similar naming convention for the first beast to appear in each nation. Compared to the creatures which spawned in their aftermath, these initial abberations were always far more massive and powerful. This is perhaps due to the fact that those most susceptibile to the transformation were already sunk deep within the mire of despair, and this intense emotion influenced a greater quantity of dynamis.
— Encyclopædia Eorzea Volume 3, p. 284