Nightmare! is a quest you can get from <Loyter> Crimson of Zanan in Vernis after getting 20000 fame. It is a level 50 quest that sends you into one of the corners of a large room and requires you to kill various yiths, Gagu, and Blood spiders.
If the player has access to the darkness arrow spell, this mission is not as hard as long as the player's pets do not attract more than one Great Race of Yith at one time, and the player stays away from the yiths' ball range (which is about the range of their Eye of Insanity as well). It is recommended that the player simply keep their riding pet if they have one and have the rest wait at town. Once this is done, simply be careful and pull the yiths one at a time, around corners if possible. The player is recommended to bring strong healing potions in order to heal while dimmed. Rods of Change Creature can make the whole quest a great deal easier, - yiths aren't as menacing after being polymorphed into a Cupid of Love.