The Coven of Nightmares

At the beginning of your end step create a Black
Aura Curse Enchantment token named
Nightmare attached to an opponent. The token
has enchanted player and "at the beginning of enchanted players upkeep they lose 1 life."
Sacrifice X Curses: chose one —
•Add X {b}.
• Return target creature with mana value X or
less form your graveyard to the battlefield.