Lanterns in the Mist
This mirror is a magic item that can be used as a prison for the unwitting. Stepan Boroi used this mirror to trap his brothers and gain the patriarch mantle of the Boroi family by making a fell deal with one of the inhabitants of the mirror- Nicasor.
The group was able travel through the mirror into the land known as the House of Night. They avoided meeting Nicasor but did destroy the heart of the Mirror. The mirror and the House of Night shattered and broke apart spewing everyone back into the Boroi manor.
A few days later it appears as though the Boroi Manor shifted back into the mirror reforming itself. They defeated the new lord of the mirror- a shadow demon at which point they were thrown back into the Boroi Manor.
The group now looks for a way to destroy the mirror completely.