Have

Exquisite Blood

Whenever an opponent loses life, you gain that much life.

Cabal Ritual

Add {B}{B}{B}.
Threshold — Add {B}{B}{B}{B}{B} instead if seven or more cards are in your graveyard.

Taste of Blood

Taste of Blood deals 1 damage to target player or planeswalker and you gain 1 life.

Maralen of the Mornsong

Players can't draw cards.
At the beginning of each player's draw step, that player loses 3 life, searches their library for a card, puts it into their hand, then shuffles.

Dark Ritual

Marauding Blight-Priest

Whenever you gain life, each opponent loses 1 life.

Leechridden Swamp

({T}: Add {B}.)
Leechridden Swamp enters tapped.
{B}, {T}: Each opponent loses 1 life. Activate only if you control two or more black permanents.

Imperial Seal

Search your library for a card, then shuffle and put that card on top. You lose 2 life.

Swamp

Vito, Thorn of the Dusk Rose

Whenever you gain life, target opponent loses that much life.
{3}{B}{B}: Creatures you control gain lifelink until end of turn.

Grim Tutor

Search your library for a card, put that card into your hand, then shuffle. You lose 3 life.

Vampiric Tutor

Demonic Tutor

Search your library for a card, put that card into your hand, then shuffle.

Sheoldred, the Apocalypse

Deathtouch
Whenever you draw a card, you gain 2 life.
Whenever an opponent draws a card, they lose 2 life.

Piranha Marsh

Piranha Marsh enters tapped.
When Piranha Marsh enters, target player loses 1 life.
{T}: Add {B}.