Marrow Bats

Flying
Pay 4 life: Regenerate Marrow Bats.

Blind Hunter

Flying
Haunt (When this creature dies, exile it haunting target creature.)
When Blind Hunter enters or the creature it haunts dies, target player loses 2 life and you gain 2 life.

Malakir Familiar

Flying, deathtouch
Whenever you gain life, Malakir Familiar gets +1/+1 until end of turn.

Gurmag Swiftwing

Desecrated Tomb

Whenever one or more creature cards leave your graveyard, create a 1/1 black Bat creature token with flying.

Kelinore Bat

Foundry Screecher

Flying
Foundry Screecher gets +1/+0 as long as you control an artifact.

Gloomhunter

Basilica Screecher

Flying
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.)

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, Vexing Pest dies, you gain 4 life.

Blex, Vexing Pest // Search for Blex

Bartizan Bats

Flying (This creature can't be blocked except by creatures with flying or reach.)

Eyetwitch

Flying
When Eyetwitch dies, learn. (You may reveal a Lesson card you own from outside the game and put it into your hand, or discard a card to draw a card.)

Sengir Bats

Flying
Whenever a creature dealt damage by Sengir Bats this turn dies, put a +1/+1 counter on Sengir Bats.

Stalking Vampire

At the beginning of your upkeep, you may pay {2}{B}{B}. If you do, transform Stalking Vampire.

Screeching Bat

Flying
At the beginning of your upkeep, you may pay {2}{B}{B}. If you do, transform Screeching Bat.

Screeching Bat // Stalking Vampire

Belfry Spirit

Flying
Haunt (When this creature dies, exile it haunting target creature.)
When Belfry Spirit enters or the creature it haunts dies, create two 1/1 black Bat creature tokens with flying.

Skeletal Vampire

Flying
When Skeletal Vampire enters, create two 1/1 black Bat creature tokens with flying.
{3}{B}{B}, Sacrifice a Bat: Create two 1/1 black Bat creature tokens with flying.
Sacrifice a Bat: Regenerate Skeletal Vampire.

Ghastly Gloomhunter

Kicker {3}{B} (You may pay an additional {3}{B} as you cast this spell.)
Flying, lifelink
If Ghastly Gloomhunter was kicked, it enters with two +1/+1 counters on it.

Dementia Bat

Flying
{4}{B}, Sacrifice Dementia Bat: Target player discards two cards.

Regal Bloodlord

Flying
At the beginning of each end step, if you gained life this turn, create a 1/1 black Bat creature token with flying.

Dakmor Bat

Blight Keeper

Flying
{7}{B}, {T}, Sacrifice Blight Keeper: Target opponent loses 4 life and you gain 4 life.

Dirge Bat

Mutate {4}{B}{B} (If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Flash
Flying
Whenever this creature mutates, destroy target creature or planeswalker an opponent controls.

Scion of the Ur-Dragon

Flying
{2}: Search your library for a Dragon permanent card and put it into your graveyard. If you do, Scion of the Ur-Dragon becomes a copy of that card until end of turn. Then shuffle.

Duskhunter Bat

Bloodthirst 1 (If an opponent was dealt damage this turn, this creature enters with a +1/+1 counter on it.)
Flying

Bat Whisperer

When Bat Whisperer enters, if an opponent lost life this turn, create a 1/1 black Bat creature token with flying.

Blighted Bat

Flying
{1}: Blighted Bat gains haste until end of turn.

Bloodhunter Bat

Flying
When Bloodhunter Bat enters, target player loses 2 life and you gain 2 life.

Grimclaw Bats

Flying
{B}, Pay 1 life: Grimclaw Bats gets +1/+1 until end of turn.

Vampire Bats

Flying (This creature can't be blocked except by creatures with flying or reach.)
{B}: Vampire Bats gets +1/+0 until end of turn. Activate no more than twice each turn.

Sengir Nosferatu

Flying
{1}{B}, Exile Sengir Nosferatu: Create a 1/2 black Bat creature token with flying. It has "{1}{B}, Sacrifice this creature: Return an exiled card named Sengir Nosferatu to the battlefield under its owner's control."