the damage is actually an ability based from Item Damage Bonus. when a unit is killed, add the ability to the killing unit. the very basic trigger for it is
Events
Unit - A unit Dies
Conditions
(Level of Necromastery for Killing unit) Greater than 0
Actions
Add DamageBonus to Killing unit
that trigger will add the damage bonus to the killing unit. but of course you will need some If-Then-Else checking to check if the unit's Necromastery is of a certain level, if the bonus is already at its max, etc.
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btw, if you post something at the World Editor Help or Triggers forum, many people will actually be able to help you. not just me. most of the knowledge i got from triggering came from there, especially users like Maker (he's really awesome XD), Pharaoh_, Adiktuz (i think this one's a Filipino too), Bribe, and many more