You want to use an ability based upon the Orb of Slow.
The Orb of Slow ability is unique in that it does exactly what you want. It gives the unit a % chance to cast a spell that you define. However, it only works with single-object-target spells. This does included AoE things like Forked Lightning and Drunken Haze that have one specific object target.
I believe that, if you have the spell that it casts take mana, it will only fire when the unit has enough mana, and it will spend that mana. Generally people make it not take mana.