Ritual of the Necromantists

When Ritual of the Necromantists enters the battlefield, exchange control of your graveyard with the graveyard of target opponent.
At the beginning of your upkeep, sacrifice a creature you control and lose 2 life, or sacrifice Ritual of the Necromantists.
You may cast creature spells from your graveyard, and may spend mana as though it were mana of any color to cast those creature spells.