Dazzling Theater // Prop Room — Duskmourn: House of Horror Promos #3p

Creature spells you cast have convoke. (Your creatures can help cast those spells. Each creature you tap while casting a creature spell pays for or one mana of that creature's color.)

(You may cast either half. That door unlocks on the battlefield. As a sorcery, you may pay the mana cost of a locked door to unlock it.)