Not only can you not modify protounit a specific unit, you can also not tell when a specific unit kills another unit, only player kills.
So yeah, it's over.
...
...or is it?
*music plays*
Age of Mythology is home to a mysterious hydra that changes appearance and attack when killing. After investigations and testings, the following conclusions have been unearthed.
First off, DragonQ's tooltip that each Additional Head gives +100% Base Damage is straight up boogoo. Hydra's ability is more like +15% Damage per kill. So after 3 kills, when the second head appears, it's not even getting half the base damage bonus.
This ability is caused by the
ExperienceUnit flag. In addition to the +15% Damage, the unit also receives +9 experience per kill, regardless of how strong the killed unit was. This experience is then used by the animation to determine how many heads to show.
Now I don't know the game code, but if I were you I would ask WarriorMario to make us a trGetUnitExperience function for the next patch.
Fun fact: Did you know there is a third unit, besides Hydra and Scylla, that increases attack with kills? The Servant. I couldn't find this mentioned anywhere, but it's true. Place a Servant in editor and his attack goes up with each kill. Their history page says they can't fight well, so I guess the poor guys must be trying to prove it wrong XD
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Skyrim... the greatest game ever... without considering Age of Mythology, of course!
Ctrl...Alt...Q...
and Brisingr-know these three things and your task here is accomplished...