3: Bladewind
The horde thinner spell. By itself it's ok with majority of units (especially the ones fielded in large numbers) that are prime target for this spell are WS3, meaning you will hit half of the models in the unit, possibly wound two thirds of that, so for an easy spell to cast you can thin a horde by a third, more if you get lucky... or slaughter them if you pull off Word of Pain first. Yeah, imagine taking a WS test with WS1 or 0.
Got this off on 20-25 saurus at a tournament recently, WS0 hit all of them but with their T4 and armor did about 8-10 wounds I think. Still awesome.
This spell is awesome against hoards. I really want to roll this spell up (or just choose it with the tome) against some hoards like empire halberdiers. Haven;t used it against undead yet but i suppose with one cast it would take a 40-50 unit of zombies or skeletons down to an easy 1 turn wipe on a charge.