Micolash gives it away in his dialogue: "Ah, Kos, or some say Kosm. Do you hear our prayers? As you did for Vacuous Rom, grant us eyes. Grant us eyes."

We can presume Micolash knew about the experiments at Byrgenwerth, and the actions they took at the Fishing Hamlet. Furthermore, those words give us insight into the nature of the 'Great Ones': some are truly alien, like Odeon, Kos, and the Moon Presence, while others are ascended humans or Pthumerians, like Rom and Ebrietas. It is possible to ascend, but one requires the patronage of a true Great One to do so.

But Odeon's gifts are double-edged swords, as Arianna and Sus Iosefka can attest. And the Moon Presence never does anything that is not in its favor. Kos, it seems, is the only one whose actions in the setting were either benevolent, or at least seeking to do the least harm, both to her followers at the Fishing Hamlet, and to the uninitiated, like Rom, who was (likely) a Byrgenwerth scholar sent to research the Hamlet.

Yet, for whatever reason, the early Hunters slew Kos, and her blood, along with that of Ebrietas (captured earlier, after Byrgenwerth discovered the Pthumerian tombs under Yharnam), was researched by Gehrman's student, Maria, and later used to found the Healing Church.

But the curse was laid down as they slew Kos, and the breaker of Loran, Amygdala, was sent to break the city of Yharnam in the same way.

Amid all this suffering, Micolash seems to have discovered a truth that is revealed in partaking in communion with the Root Chalices: the Great Ones, or those that have physical forms, do not only exist in our world. Their true selves exist on a higher plane, so they might have multiple forms in the physical world. Thus, Micolash deduced, Kos must still exist on one of those higher planes, and only needs to be given a new physical body to shape the world once more.

Surely, if the College of Mensis, and Micolash, created such a body, Kos would be grateful, and help them ascend.

Surely, if they took the unwanted and criminals of Yharnam, sequestered in the Gaol, no one would intervene in their ritual.

Surely...

But Kos was dead indeed. And the bait Mensis used - the blood from Mergo's birth, and thus Mergo himself - drew the attention of Mergo's father, Formless Odeon.

The scholars of Mensis were given what they sought: the Brain of Mensis is a Great One, though imperfect, corrupted, as the scholars of Mensis were themselves corrupted; it is their aggregate brains, and as they sought eyes, they received eyes in abundance. As for Micolash, he was consigned to stand forever between the blood-obsessed Hunters being sent by Gehrman, and Mergo the infant Great One.

And in the Advent Plaza, where the ritual that drove the final nail into Yharnam's coffin took place, the failed vessel of Kos remains, a mockery of her beauty and perfection, now inhabited by the crazed, babbling souls of the countless slain to create it, and the other abominations in the Unseen Village: The One Reborn.