Culdcept/YMMV

From the game:

 * Game Breaker: Some of the storyline and secret cards have ridiculously useful abilities, like "increase all allies HP max".
 * Cait Sith is a good example. Immune to spell cards and can't be damaged by creatures with more than a certain base attack. Set up on a max level fire space, only a few card can even stand a chance of killing it, though a scroll attack (which penetrates the defensive land bonus) will kill it in one hit.

From the manga:

 * Nightmare Fuel: Horrowitz. This Cool Old Guy had his own eyelids sewn shut and eyes on his hand for some reason.
 * The bad guys employ quite a few characters who are Nightmare Fuel incarnate. Particularly the bug guy and his ilk, who wore human skin as disguises.