It's the setting for Chapter 20, as Horace requests Zael's help to rescue his wife, Meredith. As the Chapter progreses the mysteries unfold, mainly by reading the diaries found in it by Zael. One belonged to a maid named Henrrietta who expresses her happiness about working there and says her masters (the lady of the house and the child) are good people but the man is a bit distant. The second belonged to the lady of the house who is wondering why the maids they contract leave without even taking their personal possessions or saying farewell. The third is from the house owner. This last diary reveals the last events to solve the mystery. The mansion was based on the titular mansion from the Gamecube game Luigi's Mansion.
The master of the house wanted immortality, and for that he made a pact with the vampire Nebirous, who asked in return lives. The Master offered the lives of the maids he contracted to help in the house. Not much time passed before Nebirous demanded more, so he even sacrified his wife and his child for the immortality. Nebirous still wouldn't give him the promised immortality, and eventually Nebirous takes control of the house. He then creates the Undead in it and takes the lives of anyone who enters the mansion, as Meredith did, creating the rummor of the "Man-eating" mansion.
It's unlocked in normal difficulty after completing Chapter 20. You can enter with a group of 5 members, and there will be 5 gold chests. The boss will be Nebirous. The party will receive Death (cursed) upon completion NG+ X5 Particle of the Outsider