How do you change undead-only targeting?

I know you have to use a trigger, but do you code the whole spell or can you just change the undead-only targeting?

doesnt take triggers to change targets, you just need a dummy ability and to trigger it's effects

Make a unit-targeting dummy spell, preferably based off of one that an AI would automatically use on an allied non-hero in some situation, like Rejuvenation. Give that to the hero.
Make a trigger that runs when a unit finishes casting an ability. Check to see that the ability cast is your dummy, and then check its current life and give the casting hero some percentage of that. Then, Explode or Kill the targeted unit.

K How does this work if I want a spell only target Human, Night elve or orc?

You could add in a component to check whether the unit fits certain conditions. You'll have to open up the trigger editor and play around a bit.