'Nightmare Shroud Demon? Is it a creature related to the Underworld?'
While it was not always the case, monsters with demon in their name typically had some form of ties to the Underworld.
The Underworld was a mysterious place and there was not much information know about it available to players. Even Izroth who had been to the second floor of the Amaharpe Palace Library had barely come across anything related to the Underworld.
That being said, Izroth was not completely unfamiliar with the Underworld. A while back, Izroth launched an attack on the Siren organization for putting out an assassination order on those under his protection. During that battle, he clashed against the head of Siren, Smiling Demon, who formed a contract with one of the Royal Demons of the Lemegeton, the 54th Great Duke of the Underworld, Murmur.
Although Izroth only fought with a fragment of the Great Duke and its two ministers, the power they displayed was comparable to someone in the legendary realm.
