Gray Merchant of Asphodel

When Gray Merchant of Asphodel enters the battlefield, each opponent loses X life, where X is your devotion to black. You gain life equal to the life lost this way. (Each {B} in the mana costs of permanents you control counts toward your devotion to black.)

Sedgemoor Witch

Menace
Ward—Pay 3 life. (Whenever this creature becomes the target of a spell or ability an opponent controls, counter it unless that player pays 3 life.)
Magecraft — Whenever you cast or copy an instant or sorcery spell, create a 1/1 black and green Pest creature token with "When this creature dies, you gain 1 life."

Bubbling Muck

Until end of turn, whenever a player taps a Swamp for mana, that player adds an additional {B}.

Dark Ritual

Blood Funnel

Noncreature spells you cast cost {2} less to cast.
Whenever you cast a noncreature spell, counter that spell unless you sacrifice a creature.