The waters of the black pit don't reflect light as water should. Neither does the light penetrate. It smells of rot, moss and stagnant things, and gives off a low rumbling punctuated by an occasional snap. Though the ground in this region of the forest was pitted with mires and swamps, this particular pool had always been known as "The Bog". It had no name, but the villagers of the Approaches all knew that it claimed them.
The Bog was the villagers' protection, because even the things that terrorized the forest feared the Bog. As a consequence, the villagers only gave lip service to fealty to Lord of the Approaches, and even to religion. The sole thing they really seemed to respect was that watery pit. And whatever lived deep within it.
Villagers that died left instructions for their bodies to be returned to the Bog. You didn't need to weigh bodies down when feeding them to the Bog. Corpses sank in and did not return, and the Bog consumed their souls.
The Entity of the Bog resided here for centuries, infusing everything in the area with its power. It seeped into the souls of the villagers and tied them to it.
When Prioress Merlinde arrived — sent to the region by the Church of Avacyn to teach the people the correct path of faith — she went to the Bog and looked into it using her powers. There, she found something terrible, vast, and ancient. Knowing that she could not fight it with conventional prayers or wards, she built the priory of the Nameless Angel, and dedicated everything she had to converting the people of the Approaches. But few of the villagers accepted the Angel's Blessed Sleep. However, souls began being converted, and they gave themselves to the Angel instead of the Bog.
Merlinde was finally making some progress with the people of the Approaches, when Tacenda Verlasen and he sister Willia were born and started manifesting powers. It made the people follow the Bog again, upended nearly everything she had accomplished so far.
When the girls were about fourteen, Merlinde turned the tide around again by converting Willia.