Gray Merchant of Asphodel

When Gray Merchant of Asphodel enters, 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.)

Mire Boa

Swampwalk (This creature can't be blocked as long as defending player controls a Swamp.)
{G}: Regenerate Mire Boa. (The next time this creature would be destroyed this turn, instead tap it, remove it from combat, and heal all damage on it.)

Dimir Machinations

Look at the top three cards of target player's library. Exile any number of those cards, then put the rest back in any order.
Transmute {1}{B}{B} ({1}{B}{B}, Discard this card: Search your library for a card with the same mana value as this card, reveal it, put it into your hand, then shuffle. Transmute only as a sorcery.)

Darkest Hour

Nightcreep

Until end of turn, all creatures become black and all lands become Swamps.

Sign in Blood

Target player draws two cards and loses 2 life.

Stalker Hag

Swampwalk, forestwalk (This creature can't be blocked as long as defending player controls a Swamp or a Forest.)

Nantuko Blightcutter

Protection from black
Threshold — Nantuko Blightcutter gets +1/+1 for each black permanent your opponents control as long as seven or more cards are in your graveyard.

Leshrac's Rite

Enchant creature
Enchanted creature has swampwalk. (It can't be blocked as long as defending player controls a Swamp.)

Nighthaze

Target creature gains swampwalk until end of turn. (It can't be blocked as long as defending player controls a Swamp.)
Draw a card.