'Action : Sacrifice a corrupt unit to give this unit P until the end of the turn'

Can this effect be used on an opponent's units and is there a limit to how many times it can be used in a turn? For example, could you use it everytime an opponent drops a unit into a zone with a Warpstone Excavation?

You can only sacrifice units you control. That said, you can sacrifice as many corrupt units as you want per turn with the Great Unclean One's ability.

Whilst I don't doubt the explanation, how is it that you'd know it was only for units you controlled? Does a card have to specify that opponent's units can be targetted with an effect else you assume they can't?

If you sacrificed your opponent's character, that's not really the surrender or destruction of something prized or desirable for the sake of something considered as having a higher or more pressing claim now, is it?

Are you saying it can't be that because that would be too good?

Or is 'Sacrifice' the key word here? In the sense that you can't sacrifice an opponent's unit?

Again, just trying to nail down how the ruling is got at so I can identify it in other situations

I'm saying its not a sacrifice to eliminate an opponent's unit.

I think it would be strange if you felt sacrifice as you destroyed your opponent's units while inflated the power of your awe-inspiring intestine spewing behemoth of war.

I had the same issue a while back. Go back to the rulebook & Errate - you can only sacrifice a unit 'YOU CONTROL'. Note some card effects allow you to take control of enemy units for a turn, or until they are killed.

From the FAQ: