http://www.thehelper.net/forums/showthread.php?t=57735& this is a good hero revive system (along with other things). As far as non-heroes, just change the coniditions. For other things, just change part of the trigger to your liking. I hope this is what you need (still a little foggy).
Can ya copy and paste the code of the ability? Also, origin might be hard to see, try changing it to chest and see if you see it (not sure if you already tried, but just an idea). Also, try basing it off Sphere instead. (that is saying you want an attachment, and not an ability)
edit: here is a...
You can go arcane before 64, just cast fire ball instead of arc. blast until ur 64, it won't be so bad thats its unplayable :P, that is saying you <3 arcane over frost and fire.
Ah your right, my bad, always forget the units still are counted after being dead. Thanks for catching that Jomik :D
edit: be sure to use the up to date one thanks to Jomik, it should work
Sample
Events
Unit - A unit Dies
Conditions
(Number of units in (Units in (Playable map area) matching (((Unit-type of (Matching unit)) Equal to Peasant) and (((Matching unit) is alive) Equal to True)))) Equal to 0
Actions
I'm not sure if you just wanted an event...
Go to the Object Editor menu, and go to file, export all object data, make it into a file, and then in the map you want to get it to, go to Object Editor -> file and click import all object data and click the one you just exported.