Historical Rules · 25 September 2014
Rule 702.47a
Official source text from the 25 September 2014 Rules release
Offering is a static ability of a card that functions in any zone from which the card can be cast. “[Subtype] offering” means “You may cast this card any time you could cast an instant by sacrificing a [subtype] permanent. If you do, the total cost to cast this card is reduced by the sacrificed permanent’s mana cost.”