Mettle
From NsdWiki
Typically used with: Courage.
In short: tell 'em to go spit. Mettle is a measure of temerity, fortitude, and rebellion of spirit. Mettle checks are called upon in opposition to curses and supernatural attacks. A better way to think of this skill is Vitality of the spirit; a tangible supernatural toughness.
Difficulty
Mettle checks are always in opposition to another roll or effect, so there are no set difficulties.
Opposed Rolls
Mettle checks are regularly used to oppose some Magic Spells and similar effects.