Sounds like a cool idea! Here is how I'd approach it:
First, you'll want to trigger the damage for Overhead-Smash. This will make the detection process a lot easier. For the sake of simplicity, let's say that Overhead-Smash just deals some damage (you can modify the trigger however you'd like):
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Overhead Smash
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Events
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Unit - A unit starts the effect of an ability
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Conditions
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(Ability being cast) Equal to Overhead Smash
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Actions
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Unit - Cause (Triggering unit) to damage (Target unit of ability being cast), dealing 500.0 damage of attack type Normal and damage type Normal
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If (All Conditions are True) then do (Then Actions) else do (Else Actions)
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If - Conditions
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((Target unit of ability being cast) is dead) Equal to True
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Then - Actions
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-------- Activate ability -------
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Else - Actions
The trigger above just detects when Overhead Smash is used, it deals damage, and then checks if the unit is now dead. I can't guarantee whether this will work (I don't know whether wc3 flags units for death immediately after damage has been dealt, you should test it).
The part where it says "Activate Ability" is the main portion of concern. What you do there is really dependent on how you want to structure your ability. If you want it to be similar to Victory Rush, then you would probably start a timer (for about 8 seconds, for example). This would essentially be the user's "window" in which he can use the ability. If he uses the ability in that window, deal damage and heal the unit.
I'll wait for your response. We need a few more details on how
you want your spell to work. For example, you could make it so that "Victory Rush" is activated when the caster gets a killing blow. Or you could make it so that it is activated only when the caster gets a killing blow with Overhead Smash. Orrr you can make it active at all times, but have it only restore HP if used after a killing blow from Overhead Smash. There are lots of permutations.
