Unfortunately, you cannot detect buffs when a unit dies, because all buffs are removed from a unit BEFORE the Event "A unit Dies" happens. At least this was the case last time I tried doing something like this.
Solutions:
Use a Damage Engine to detect when a unit takes lethal damage.
Trigger the Aura yourself using a periodic interval and unit groups. A unit dies -> if unit is in RaiseDeadUnitGroup = True -> Do stuff.