Well I think canibalize should work as a base spell, since you can target a corpse. You'll need to modify duration to 0.01 or something really short because it's a channeling spell.
Triggers... hmm...
create one unit type of targeted unit(dunno if this is correct) for owner of triggering unit at position of targeted unit.
add 20 seconds expiration timer to last created unit
remove targeted unit from the game
add special effect
maybe change tinting color of the created unit.
of course you'll need to get rid of leaks for locations and special effects, look at some triggers and you'll understand how to do it.