The target of this spell is a creature who the caster saw become injured (lost at least 1 hit point) within the last 10 minutes. The caster mentally chooses the wound and focuses the spell energy upon it, festering the wound and turning it into a critical hit. (If the wound was caused by something with no critical modifier, treat it as if the modifier was x2.) Any special effects of the implement that caused the wound are ignored.
Example: An ogre hits Tordek with its vorpal battleaxe and rolls 1d8, causing 6 points of damage. With the help of a spectral hand, Amberlei focuses on this wound and casts fester wound. The critical damage (x3) is applied, causing 12 points of new damage. The vorpal effect does not kick in, so Tordek gets to keep his head.
The caster need not be able to touch the specific wound, only the target creature. Creatures immune to criticial hits are immune to this spell. If the wound has been healed through regeneration, magic, or some other method, the spell fails (assume that wounds heal in the order they were received). Wounds that were caused by normal (rolled) critical hits are immune to this spell. This spell cannot be used twice on the same wound.
Material component: A single, whole bone, which must be broken into two pieces as the somatic component of the spell.