finished casting = reincarnation would that do?
Does not work either. Passive abilities usually don't have cast events. The only reason for GUI unit event to exist is to emulate events, that are not possible through normal events, but are very useful.
Or trigger that checks if a unit got attacked then checks if its hp is below 1
A unit is attacked event fires, when the attacker starts the attack animation, so you can still cancel it and no actual attack happens. You can use: attack-stop-attack-stop.. very quickly to fire multiple attack events in one second.
You usually use a Damage Detection System (DDS) to detect if a unit takes damage, because it's much more reliable. However this still does not work, because units can die without taking damage. Examples are instant kill skills (at least I think they don't deal damage): death pact, dark ritual, transmute
And negative regeneration: phoenix, unholy frenzy
If it's going to be replaced anyway, why even use the reincarnation ability? Let the unit die normally which is very easy to detect with triggers, play the reincarnation special effects, spawn the new unit after a delay, and remove the old unit's corpse.
This is a good solution, unless it's important, that the unit does not die. As I said reincarnated units do not fire the death event, so it won't be counted as a kill and it won't give gold or experience. Same problem with the hydra ability.
So basically if you actually wan't the unit to die (give gold, drop items, give experience, fire death event) and after that replace it you can use hydra ability or Kaiyuu's solution.