Chord of Calling is a rare Instant card from Ravnica: City of Guilds (RAV), collector number 156. Current market price: $7.32 (foil $72.42), TCGplayer market prices via Scryfall.
Convoke (Your creatures can help cast this spell. Each creature you tap while casting this spell pays for {1} or one mana of that creature's color.) Search your library for a creature card with mana value X or less, put it onto the battlefield, then shuffle.