I have a Champion of Stray Souls. It says, in short, that I can trade any number of creatures (sacrifice) for any other creatures in my graveyard. It does not say creatures in my hand or on the battlefield, so my question is: Can I sacrifice creatures in my deck or in my hand (or just my hand but not my deck, etc.) to trade in for creatures in the graveyard, or just creatures on the battlefield?
You can only sacrifice creatures that are on the battlefield and you control.
From the comprehensive rules:
701.14a To sacrifice a permanent, its controller moves it from the battlefield directly to its owner’s graveyard. A player can’t sacrifice something that isn’t a permanent, or something that’s a permanent he or she doesn’t control. Sacrificing a permanent doesn’t destroy it, so regeneration or other effects that replace destruction can’t affect this action.
A permanent is a card on the battlefield, such as creatures, lands, artifacts, etc.. A card that is not on the battlefield can never be a creature (land, etc), only a creature card (land card, etc.).