In Exalted, the Yozi Hegra, the Typhoon of Nightmares, produces hallucinogenic weather...or acid rain.
Malfeas, the Demon City, has a charm called Nightmare Fugue Vigilance, which means he doesn't have to sleep. That's right, Malfeas is The City That Never Sleeps.
In the Unhinged joke set of Magic: The Gathering, all creatures with the subtype "donkey" have fractional numbers for their power and toughness. Why? Because they're half-assed. Yes, this was confirmed as intentional.