Blackstone Altar

Deep in the Lowerdark there is a section of barren cave approximately 150 feet tall, and 300 ft in diameter. There is no fungi, or any vegetation that grows in the cavern. In the exact middle of the cavern lies the only source of light: A bright cluster of luminescent rock, shining a spotlight directly below onto a pillar, standing approximately 30 feet high and about 20 feet in diameter, made of oily black stone. While the base, and sides of the pillar appear to be natural, the top forms a smooth perfect circle, and along the sides is a winding staircase carved out of the pillar leading to the top. At the top of the pillar, in the dead center, is a black stone altar that forms a wide, but shallow bowl. 
Creatures that get close enough to be affected by the altar feel a sense of calming content, which convinces them that nothing is wrong, or out of place here. Nearby creatures with lesser intelligence & willpower (constitution) feel compelled to reach the top and either kill themselves (if they are weak enough), or self harm, and bleed into the shallow bowl. 
There are no identifying markings of any kind, and no bones, or corpses nearby. The blood that collects in the bowl seems to be absorbed into the oily black stone. Nobody knows who/what made the Black Stone Altar, or when it was constructed - it has simply always been there. People have theories of what happens to the bodies of those who sacrifice themselves, but no one has ever seen anyone, or anything show up to dispose of the corpses. The people in the Underdark fear it, and avoid going near it whenever possible.

Type

Shrine