The Shadows of Falcon's Hollow
Bristle feels confident that the altar is related. This demon is mostly worshiped by evil werewolves, so perhaps the undead werewolf was one of her followers, if she had a cult here. It looks like a sacrificial altar used for blood sacrifice, and such actions certainly have a tendency to leave metaphysical stains on the world.
Meanwhile, those continuing down the passageway find that there are numerous cracks in the stone floor. Far below, in the dark, you can hear...rushing water. An underground river, it would appear, some sixty feet down. Crossing over the water are multiple arches of stone, natural bridges of some sort, criss-crossing the river. The water looks dark, and fast, and cold.