I have a Shadowborn Demon and I was a little curious about the description of it. It specifically says "At the beginning of your upkeep, if there are fewer than six creature cards in your graveyard, sacrifice a creature."
So what I was wondering is, do I have to sacrifice my own creature? Or can I try and keep less than six creatures in my graveyard and sacrifice an opponent's creature every upkeep? Since it does specifically says "a creature" — it does not say "your creature" or "a creature you control." Most cards that have a sacrifice involved are extremely specific about that and will say "your creature" or "a creature you control" if it wants you to sacrifice your own creatures.