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.)

Forest

Nest of Scarabs

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

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.

Crocodile of the Crossing

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

Golgari Guildgate

Golgari Guildgate enters tapped.
{T}: Add {B} or {G}.

Festering Mummy

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

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.)

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.)

Baleful Ammit

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

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.

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.