I was on a wall when sieging army threw down their weapons and turned away. Some of them were weeping. The battle was over - for them.
Any creature subject to mind spells will become so dispirited that they will always act after all creatures not suffering from bad morale, and will have their damage reduced by 20%.
The effects of Mass Sorrow are identical to the single-target version, and like the smaller version, will not take effect until the start of the turn after it is cast. Also, like Sorrow, it will not affect elementals, constructs, nor the undead.
This spell is an excellent start to a battle, as the penalties it causes will ensure that you go first, and that your enemies will strike less forcefully. If you wait to cast it until the battle is mostly decided, your enemies may already be suffering from reduced morale due to circumstance, and the spell will have less effect.