I can think of 3 different approaches to triggering this:
1. Easy but inefficient and inexact: use Breath of Frost instead. Use the event that the hero casts the spell, wait a short duration, then pick every unit in the map (or in a radius from the caster) on the condition that they have the buff placed by the modified Breath of Frost spell. Then, order a dummy to Curse every picked unit.
2. Easy but requires an additional system: have the hero only cast a dummy spell that does nothing but targeting. Then, at the spellcast event, have a dummy unit cast the real Breath of Fire from the position of the hero. Using a damage detection system (such as
the one I have in the Spell section
), detect when that dummy causes damage; these will be the units hit by the Breath of Fire. Have another dummy Curse them.
3. Complicated but has no other requirements: trigger the wave spell, by indexing (
tutorial here) each spellcast and periodically moving a point in the direction that the spell was cast and picking all units around it. Then, have a dummy unit Curse the picked units.
I'd suggest deciding on one of those approaches, and then giving it a try. If you can't figure it out, ask us again here for advice.