This has come up a couple times now in some of my games with the Dark Eldar during the combat phase. My opponent (IG) and I (DE) were not too sure when this card is played if it affects the current battle and units present on the planet being fought over or if this affects any unit on any planet that is in play?
The card states Combat Action : Your opponent must sacrifice an army unit, if able."
It seems to me that this would just affect the current enemy units on the current planet being fought over since this is being used as a combat action during that specific battle.
But this could also mean any unit on any planet, but I could be wrong.