Blessing of Belzenlok

Target creature gets +2/+1 until end of turn. If it's legendary, it also gains lifelink until end of turn.

Swamp

Aphotic Wisps

Target creature becomes black and gains fear until end of turn. (It can't be blocked except by artifact creatures and/or black creatures.)
Draw a card.

Darkblast

Target creature gets -1/-1 until end of turn.
Dredge 3 (If you would draw a card, you may mill three cards instead. If you do, return this card from your graveyard to your hand.)

Coat with Venom

Target creature gets +1/+2 and gains deathtouch until end of turn. (Any amount of damage it deals to a creature is enough to destroy it.)

Festering Goblin

When Festering Goblin dies, target creature gets -1/-1 until end of turn.

Arcane Signet

{T}: Add one mana of any color in your commander's color identity.

Horobi, Death's Wail

Flying
Whenever a creature becomes the target of a spell or ability, destroy that creature.

Charcoal Diamond

Charcoal Diamond enters the battlefield tapped.
{T}: Add {B}.

Eye Gouge

Target creature gets -1/-1 until end of turn. If it's a Cyclops, destroy it.

Crypt Ghast

Extort (Whenever you cast a spell, you may pay {W/B}. If you do, each opponent loses 1 life and you gain that much life.)
Whenever you tap a Swamp for mana, add an additional {B}.

Cruel Feeding

Strive — This spell costs {2}{B} more to cast for each target beyond the first.
Any number of target creatures each get +1/+0 and gain lifelink until end of turn. (Damage dealt by a creature with lifelink also causes its controller to gain that much life.)

Defile

Target creature gets -1/-1 until end of turn for each Swamp you control.

Bubbling Muck

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

Sol Ring

Festering Newt

When Festering Newt dies, target creature an opponent controls gets -1/-1 until end of turn. That creature gets -4/-4 instead if you control a creature named Bogbrew Witch.

Festering Mummy

When Festering Mummy dies, you may put a -1/-1 counter on target creature.

Alchemist's Gift

Target creature gets +1/+1 and gains your choice of deathtouch or lifelink until end of turn. (Any amount of damage a creature with deathtouch deals to a creature is enough to destroy it. Damage dealt by a creature with lifelink also causes its controller to gain that much life.)

Dark Ritual

Disfigure

Target creature gets -2/-2 until end of turn.

Borrowed Malevolence

Escalate {2} (Pay this cost for each mode chosen beyond the first.)
Choose one or both —
• Target creature gets +1/+1 until end of turn.
• Target creature gets -1/-1 until end of turn.