Spread the Sickness

Destroy target creature, then proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Decimator Beetle

When Decimator Beetle enters, put a -1/-1 counter on target creature you control.
Whenever Decimator Beetle attacks, remove a -1/-1 counter from target creature you control and put a -1/-1 counter on up to one target creature defending player controls.

Festering Mummy

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

Forest

Jungle Hollow

This land enters tapped.
When this land enters, you gain 1 life.
{T}: Add {B} or {G}.

Defiant Greatmaw

When Defiant Greatmaw enters, put two -1/-1 counters on target creature you control.
Whenever you put one or more -1/-1 counters on Defiant Greatmaw, remove a -1/-1 counter from another target creature you control.

Nest of Scarabs

Whenever you put one or more -1/-1 counters on a creature, create that many 1/1 black Insect creature tokens.

Exemplar of Strength

When Exemplar of Strength enters, put three -1/-1 counters on target creature you control.
Whenever Exemplar of Strength attacks, remove a -1/-1 counter from it. If you do, you gain 1 life.

Grim Affliction

Put a -1/-1 counter on target creature, then proliferate. (Choose any number of permanents and/or players, then give each another counter of each kind already there.)

Baleful Ammit

Lifelink
When Baleful Ammit enters, put a -1/-1 counter on target creature you control.

Swamp

Hapatra's Mark

Target creature you control gains hexproof until end of turn. Remove all -1/-1 counters from it. (A creature with hexproof can't be the target of spells or abilities your opponents control.)

Crocodile of the Crossing

Haste
When Crocodile of the Crossing enters, put a -1/-1 counter on target creature you control.