The most useful spell.

Massive confusion. The entire enemy army doesn't attack or retaliate. Combined with the "Sharp Mind" skill, it's an incredibly powerful combination. And with the "Sandro's Cloak" artifact, it's even more devastating. For example, against mages, you can immediately disable three stacks of ranged units: gremlins, mages, and titans. Of course, if you're playing against a Necromancer, confusion is useless, unless you have Sandro's Cloak.