Mirkwood Bats

Flying
Whenever you create or sacrifice a token, each opponent loses 1 life.

Toxic Deluge

As an additional cost to cast this spell, pay X life.
All creatures get -X/-X until end of turn.

Ribtruss Roaster

Devour 1 (As this creature enters, you may sacrifice any number of creatures. This creature enters with that many +1/+1 counters on it.)
At the beginning of your end step, create a number of 1/1 black and green Pest creature tokens equal to the number of +1/+1 counters on this creature. They have "When this token dies, you gain 1 life."

Search for Blex

Look at the top five cards of your library. You may put any number of them into your hand and the rest into your graveyard. You lose 3 life for each card you put into your hand this way.

Blex, Vexing Pest

Other Pests, Bats, Insects, Snakes, and Spiders you control get +1/+1.
When Blex dies, you gain 4 life.

Blex, Vexing Pest // Search for Blex

Pest Rescuer

At the beginning of each upkeep, if you don't control a Pest creature token, create a 1/1 black and green Pest creature token with "When this token dies, you gain 1 life."
If you would gain life, you gain that much life plus 1 instead.

Blech, Loafing Pest

Whenever you gain life, put a +1/+1 counter on each Pest, Bat, Insect, Snake, and Spider you control.

Essence Warden

Whenever another creature enters, you gain 1 life.

Pest Infestation

Destroy up to X target artifacts and/or enchantments. Create twice X 1/1 black and green Pest creature tokens with "When this token dies, you gain 1 life."

Beledros Witherbloom

Flying
At the beginning of each upkeep, create a 1/1 black and green Pest creature token with "When this token dies, you gain 1 life."
Pay 10 life: Untap all lands you control. Activate only once each turn.