Polluted Cistern // Dim Oubliette

(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.)

Polluted Cistern

Whenever one or more cards are put into your graveyard from your library, each opponent loses 1 life for each card type among those cards.

Dim Oubliette

When you unlock this door, mill three cards, then return a creature card from your graveyard to the battlefield.