Instant, XGGG (3)
Rare
Convoke (Each creature you tap while casting this spell reduces its cost by {1} or by one mana of that creature's color.)
Search your library for a creature card with converted mana cost X or less and put it onto the battlefield. Then shuffle your library.
Illus. Heather Hudson

10/1/2005: You announce what X is first, and then announce which creatures you're tapping to reduce the cost to cast the spell.
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Classic (MTGO)
Legal in Commander
Legal in Modern