While effects like DEF- and ATK reduced exists in the game as separate debuffs, Weakened is the combination of both effects.
This debuff is inflicted by Stone Fist's Fist of Justice. It's a strong debuff that can turns heroes into sissies because Weakened targets not only take more damage but will also deal less damage to other heroes. The strength of the debuff depends on Stone Fist's skill stat and loweres target's ATK und DEF by more than -70% at level 80.
This effect is very useful in a team that cripples the opponent's attack or defenses, making them deal less damage overall and become more susceptible to hard-hitting attacks. After inflicting this debuff, it is common to unleash multiple attacks on a single target to ensure that the reduced defense aid in dealing more damage.