Reapertale: a summary.
As stated before, some AUs seem to run in different formats. "Reapertale" is another such AU, but rather than running as a soap opera, this Tale seems to play out like Greek mythos, positioning monsters as gods.
Toriel, goddess of life and warmth, gave breath to the earthly realm and sprouted humankind... but they were not yet mortals, and as they continued to propagate, so did conflict among them, and the balance of the earth tilted. His majesty Asgore, god of the sky from which he and the other first gods were born, and to which they would return if ever they passed on, turned to Gerson, god of wisdom, for answers. And Gerson, in turn, commissioned Gaster, god of the arcane, to conjure a counterbalance. Together, they used the power of darkness to invent the gods of death– Sans and Papyrus. Their conception deeply offended her majesty Toriel, and she secluded herself on the earth to swear off any further creation.
Though life would continue, sterility and stagnation would break out in the goddess' absence, and humanity's faith in the gods would falter. This brought on the loss of the god of hope– Asriel– as he was reaching to save Chara, a devoted acolyte, from tragedy. He was too late, and when his stardust fell into a pool of their blood, he would be reborn as an emissary of despair... again as a golden flower.
As it would happen, Chara would be the reapers' first job; but the gods of death were still newly-formed, and the dark magic used to conjure them still leaked from their forms and their scythes. Thus, only a moment's hesitation was enough for Sans' essence to corrupt Chara's SOUL, which was already embittered by the betrayal of mortal and immortal alike. They would escape, adopting the role of a demon in the following centuries.
In his effort to find Chara, Sans instead found Toriel's hidden sanctuary. The two were confrontational at first, but soon it became clear that Sans liked finding somewhere that allowed him reprieve from his thankless job, being the brother in charge of collecting the SOULs of the damned and the restless. In secret, they grew close...
...and then, claiming a role as harbinger of entropy, Chara too found Toriel. Though a god, they struck her down using a scythe stolen from Sans, igniting an Era of Corruption– an outbreak of strife, stagnation and stillbirth.
Undyne, goddess of war, was next to find what was left of the fallen god's hideaway, and confronted Sans before she could see could see that he too was grieving. Papyrus managed to intervene and calm Sans, stopping the spread of his necrotic influence, redirecting Undyne's attention to seek guidance from the father gods.
Of the first gods, Asgore was still in mourning, Gerson had gone quiet, and Gaster had gone to consult an all-seeing mirror. The mirror could show him everything– past present and future– but Chara had found it first, clouding its vision to lure him closer. They trapped him in it, and they shattered the mirror, scattering him and all memories of him with its shards.
In an effort to buy more time to rediscover life, the goddess of knowledge... Alphys... would be tasked with finding alternatives for creation. Her... first attempt involved the willing sacrifice of other gods, but these efforts would be in vain. The second attempt involved making a new vessel for a prematurely disembodied SOUL, and so... Mettaton would be created as a golem, embodying self-love.
What remained of the slain goddess was eventually detected by the opposing reapers at the same time, but Sans was thankfully faster than Chara and took her SOUL into hiding within his and Papyrus' own realm. He procured a stained-glass flower she'd once made for him after his touch withered the live white lily she previously tried to hand him, and he sacrificed the gift to bring her back with the essence of hers it had preserved. Thus, her restoration was secured.
But it would not be until the emergence of Frisk, demigod of mercy, that Chara would be cured of corruption and instead represent faith, and the balance of their world would finally stabilize.
...Seeing that mirror for myself would make my job so much easier if it were still intact...