I used the orb of annihilation ability, it is an aoe arrow ability, just make sure the unit you give it to has the "missle (splash)" weapon type and set the units aoe range to how big you want the aoe to be.
or just base it off the Firelords spells he got one passive initate or something is the name.
It will atk a unit give it a buff when the unit dies he will do AoE Damage.