Typically used with: Courage.
In short: "what curse?". 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.
Mettle checks are always in opposition to another roll or effect, so there are no set difficulties.
Mettle checks are regularly used to oppose some Magic Spells and similar effects.