Nightmare! (Will): replaces Acting, Area Knowledge (Land of Dreams), Brainwashing, Brawling, Disguise, Dreaming, Enthrallment (all), Exorcism, Hidden Lore (Land of Dreams), Hypnotism, Interrogation, Intimidation, Mimicry, Observation, and Psychology for all rolls done within a dream. It does not give the ability to invade other people’s dreams, but will allow rolls to manipulate another’s dreams from the inside.
This is not one of the ‘nice’ wildcard skills. It is typically only known by demons and other malevolent supernatural entities who wish to indulge in torture via dreams. In fact, demonstrating a knowledge of Nightmare! is an excellent way of attracting the homicidal attention of benevolent, or even strictly neutral, supernatural entities. In fact, the neutral entities might end up being even more homicidal, out of sheer enlightened self-interest: the typical benevolent supernatural entity’s response to a blatant use of Nightmare! usually includes the term ‘blast radius.’
Only tangentially related, a follow-up to the discussion about the Black Death. It’s a pretty good overview.
https://aeon.co/essays/what-caused-the-black-death-and-could-it-strike-again