The entire lower level of the shrine is clearly much, much older than the house above it. The walls were once covered in bas relief, but the stone has worn away and the carvings have become unrecognizable.
In a niche one end of the cramped corridor, the extrusions of stone seem to reach out from the wall and almost cradle a glowing ball of hellish red light which floats in midair.
Those who dare to lean close to the ball will discover that – at a distance of a few inches or so – they can actually peer into it (or perhaps, more accurately, through it). Doing so is like peering through a heavily distorted lens into a volcanic hell. (The Hell cyst is actually a pin-prick of Hell co-located onto the Material Plane. There is no way to traverse or communicate through the event horizon of the cyst, but the cyst is nevertheless a literal manifestation of Hell on Earth and can be used as the focus for a number of demonic rites.)
A large mirror with a gold frame stands at the far end of this chamber. Anyone seeing themselves reflected in the mirror is bathed in purplish flames which deal 1d6 damage per round to most creatures, but heal 1d6 damage per round for fiends.
Noble Sigil: On the back of he mirror is carved a small rune. A DC 13 Intelligence (History) check reveals that his is a Waterdhavian noble sigil; a practice that was once common in Waterdeep, but which died out over a century ago. A DC 16 check identifies this specific rune as indicating that the mirror belonged to the Cassalanter family.