Defile

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

Horobi, Death's Wail

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

Tragic Slip

Target creature gets -1/-1 until end of turn.
Morbid — That creature gets -13/-13 until end of turn instead if a creature died this turn.

Whisper of the Dross

Target creature gets -1/-1 until end of turn. Proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Sol Ring

Arcane Signet

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