Zoltan in Heroes 5

If enemy hero casts any spell, it can be blocked in his or her spellbook making it impossible to cast that spell again in this combat. The probability of blocking increases as hero gains new levels. The higher the spell's circle, the harder it is to block it.

Zoltan is one of the many Undead leaders who views mages as their true enemies. To better know them, Zoltan studied deeply in the arcane magics of the Silver Cities. As a result, he can combine his own Necromantic skills with arcane ones to a particular effect : once a spell is used by an enemy that knowledge "dies" as long as Zoltan is on the battlefield.