Weaken is a Status Effect that can be applied to a Monster or a Building. It triggers when the Building/Monster has their next action, where upon it will reduce the damage of their next attack.
Weaken can be removed by having Strengthen applied to a Monster that has been weakened. Otherwise, it will be removed after the Monster has their next action. If the action the Monster takes is not an attack, weaken has no effect.
Buildings
Poison HQs will apply the Weakened Status Effect to all Active Monsters.
Poison Defense Buildings have a chance that they will apply a Weakened Status Effect to a Monster.
HQs
Category HQs with Weaken not found
Defenses
Category Defenses with Weaken not found
Monsters
Category Monsters with Weaken not found