The wither spell is broken FYI.  It works I'm presuming as intended against single big monsters.  When its cast on units with more than 1 individual, it automatically kills that entire unit.

I was playing on the highest difficulty level and had the AI's turned up to the maximum.  Before too long, the AI is creating really nasty military units.  The wither spell is supposed to reduce the max hp's and offense by a certain percentage.  Its pretty usefull on the more difficult levels and its nice to cast on big Ogres or whatever.  However, if cast on a unit that has like 500 hps combined between all of the units, you can one shot each unit and completely kill it with Wither.