To the right of the Grand Cathedral and down the hill, there is a door beneath a lesser Amygdala, and beyond is Yahar'gul, the Unseen Village. Within the village, the entire population of Yahar'gul appears to have been killed in the process of a ritual performed by the School of Mensis, with many corpses melded with the walls or cowering in isolated corners. It is possible the ritual that killed them was responsible for summoning The One Reborn, which now resides in the chapel.
Yahar'gul was originally an experiment by the School of Mensis to see if it was possible to create a Great One through a human. They tried multiple times, and failed each time, with Darkbeast Paarl being one of these failures. Finally, they succeeded. They created The One Reborn. However, it led to the gruesome, painful demise of all of Yahar'gul's inhabitants, including the researchers. And to make things more ironic, it wasn't fit to be considered a Great One. The School of Mensis quickly left Yahar'gul, leaving The One Reborn to wander aimlessly in what was left of it.