Nice! For you, it sounds like Ceresa is a nasty evil sorcerous that you need some spell immunity against Ceresa before trying that hero stunt again. 

Cheers! 

Level 3 - Death Spellbook

Wither - Reduces the strength and constitution of target foe by 25% for 3 turns.

(Actually it reduces the attack and HP of the unit, so the description is off a little bit).

They shouldn't have died unless they were already hurt, if so then she made them shrivel up like a raisin.

I have this same issue in 1.19b.  I went into a battle where the enemy had armies and I didn't yet.  I cast wither on their army units, and boom the entire army (which had full hit points) died with one cast of wither.  I noticed the word wither appears like 3 or four 4 times above the units head with a single cast, which seems a bit odd.

You are right that the description is off. The truly strange part, which is probably causing the problem, is that, instead of the spell doing a damage effect like curHealth or DefendableDamage,  the spell is actually adjusting the unit's overall hit points directly and thus is probably being applied per unit. As far as I can tell, when doing an AdjustUnitStat by value, the game will usually try and apply all relevant bonuses. So in the case of wither, it is getting the values correctly, but then applying them per unit. In general, this spell should probably use the multiplier tag with 0.25 instead of a calculation of value since it does not appear to scale via another stat.