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 harm. A better way to think of this skill is Guard of the spirit; a tangible supernatural toughness.
Repelling the Supernatural
You can make a Mettle check to stave off a supernatural assault on your being. Many such hazards lie along the road to adventure: ancient curses, ki manipulation, spirit possessions, and corrupting jewelry forged by evil entities. The more irresistible the harm, the higher the DL.
The Hands of Fate special power describes a killing touch which is opposed by a Mettle check (see Chapter 9: Special Powers). If the killing touch lands and damage is dealt, you must make a Mettle check against a DL of 5 plus the amount of damage you took. If you fail this check, you die instantly.
Most stygian spells can be overcome with a Mettle check, for instance Steal Strength and Inflict Pain. In this case, you may add any points in Magic Defense to this roll. The mage makes a Casting Roll. You make a Mettle check. If your result meets or exceeds the Casting Roll, you elude the effects of the spell.