It's a missable trophy.
This is part of the main quest "Scenes From a Marriage"
You need to provoke with Aard or your sword the five nightmares standing still in the room while the one on the move is attacking you. When the six nightmares are attacking you defeat all of them and the trophy will pop up.
You can make a double save and do it another time, lowering the difficulty if too much hard to achieve at the moment.