Gift of the Necromancer

At the beginning of you upkeep put two colorless Sarcophagus artifact token into your graveyard, it has "{2}{b} Exile Sarcophagus from your graveyard: Put a 2/2 black Zombie creature token onto the battlefield".
Whenever you exile a Sarcophagus from your graveyard, add {b} to your mana pool, this mana doesn't empty from your mana pool as phases end.