Arkham Horror/YMMV


 * Badass: Potentially you. What else do you call the monster-fighting rich girl riding a motorcycle and armed with an enchanted shotgun?
 * Demonic Spiders, Goddamn Bats: Oh lord, where to begin? The Hound of Tindalos is a good place to start, as it's one of the few monsters that can attack you in any location, can move anywhere on the board, and if playing a low sanity character (poor Monterey Jack), it's almost impossible to fight as it will just nuke your brain instantly.
 * Plain ol' ghosts from the main game also count. They're immobile, not to mention immune to physical attack and have hefty penalties on Will, Fight and Sneak checks. In the wrong place against an ill-equipped character, they are a formidable roadblock.
 * That One Boss: Great Cthulhu himself. His cultists effect your sanity like most other monsters. Everyone permanently loses a point of maximum health and sanity with him in play. In the final battle, he regenerates a doom token after every attack.