This spell does not work, in fact it messes up a tactical battle. After casting mass berserk it attempt to cast mass wither on opponent. However, my sov gets activated with a name "Wither" and without any options active (in task bar). First time i was forced to auto-resolve battle, but later i discovered that pressing any of hot-keys on keyboard triggers corresponding (invisible) spell of my sov and then the battle continues normally.

Here is a link to the save, just hit an end turn and you will be attacked (by the way you can also see that the battle takes place in a city, but the city does not appear on the tactical screen and my milita is missing from a fight - so three bugs in one shot):

http://dl.dropbox.com/u/16401819/FE/Bug.EleSav