Nightmare mode is designed to be unfair and is not intended to be winnable although very competent players may be able to do so. Players who take up the challenge of nightmare mode will have their score increased by a significant factor. Some of the changes induced by nightmare mode are very obvious to the player while others are much more subtle.
Golems lose their immunity to stunning.
Vampires can never resist light. Monsters all start awake and can never be slept, scared or confused. Monsters' hitpoints are doubled and each monster receives a bonus of between 5 and 10 to their base speed. Monsters gain a random chance of resisting a magical attack even if they do not ordinarily have the required resistance. Monsters' hitpoints regenerate more quickly.