Living foes damaged by your spell are also weakened.
Benefit: You can alter a spell that deals damage to foes so that any living creature that is dealt damage also takes a –4 penalty to Strength for 1 minute. Strength penalties from multiple spells enhanced by the Fell Weakening feat do not stack.
A fell weakening spell uses up a spell slot one level higher than the spell’s actual level.
Source: Libris Mortis, page 27.