You will need some offensive auto-cast ability as a base (e.g. Parasite, Curse). Detect when a unit casts this auto-cast spell, create a dummy for that player, give it a Firebolt and order the dummy to cast the spell on the target.
Then detect via triggers when unit takes damage from the Firebolt/from the dummy and deal AoE damage.
As for the AoE damage + burn, you could use a modified Rain of Fire ability, as that applies damage + burn per wave, so you could modify it to just send a single wave and hide the fire rain effect