At the end of the maze, you will warp to the boss room. Once the party leader steps forward Nidhoggur's Shadow will spawn at the center of the room.
Approximately every 3 min while fighting Nidhoggur's Shadow, the entire party will be warped to 1 of 4 side rooms. They will receive several Status Effects that last for 30 seconds and ignore all resistances/immunities. 3 Ryncho and 2 Phylla will spawn in the side room with the party, they must be defeated before the party can leave the room.
Southwest (red) room: HP-75%, Bleeding Ailment (icon does not show)