As you approach Nightmare for the final encounter, tortured spirits will spawn along the sides of the room. These spirits start out neutral, but once aggroed they will run up close and attack. They're very weak, so if you do accidentally pick one up it can easily die to AoEs. Stand at the back of the room by the portal to avoid aggroing them. Nightmare has a very powerful frontal AoE, so the other two players should stay behind him, but close enough for the tank to pick up any adds without having to move. Interrupt his inductions or he will steal the targeted player's color and hamper their movement.
Nightmare also awakens a tomb and its respective guard at certain morale thresholds, so unless you can win a DPS race against an increasingly large swarm of tomb spirits, switch DPS to the guard and then the tomb to take them out before the next pair activates. Bring disease potions, as power drains can be an issue and the fight is a long one.