Sister Efner’s descent began not with a sudden act of malice, but with a quiet, erosion of faith in the light she had spent her life serving.

The Catalyst: The final descent was triggered by the loss of her youngest novice, a girl whose death was so senseless it shattered Efner's last tether to the light.

He looked up, his eyes clouding over as if a veil had been drawn across them. "Ah, Sister Efner," he murmured, setting down his tools. "A tragic figure, indeed. She was once a member of our community, a devoted soul with a voice like an angel. But it was said that she became enthralled by the Echoes of Elyria."

The sisters began to fear Sister Efner, to whisper among themselves about the change in her. They didn't know how to reach her, how to bring her back from the brink of darkness. And as the days turned into weeks, Sister Efner's fall continued, unchecked, into the abyss of shadows.

Efner does not fight him. She asks: “Have you ever watched someone die of the shaking plague for forty days?”
He hesitates. She offers him a choice: be the vessel for all remaining diseases in the colony, and die in one night of holy agony, so that fifty children may live.

that traded in hope while feeding on the desperate. The "Darkness" wasn't evil; it was the cold, hard clarity that everything she had sacrificed was for a lie. The Final Snap