The Swarm is an unlockable passive item in The Binding of Isaac, added in the Repentance DLC.

On pickupSpawns 8 orbital flies.

While heldEach time one of the flies blocks a projectile, it will turn into a Blue Fly.

An additional orbital fly is spawned upon clearing a room, up to a maximum of 16 at a time.

This page shows the latest information as of the new Repentance+ (Online Co-op) DLC.
To see information about this item for the previous Repentance version, check this page instead.

Defeat The Beast as Tainted ???.

This item contributes towards the Beelzebub transformation. Collecting 3 items from this set will grant Isaac flight, converts small enemy flies into Blue Flies, and permanently charms larger enemy flies.

The Swarm can be found in the following item pools:

The following tags are applied to this item:

The following special synergies and interactions for The Swarm have been documented.

* Note that not all possible synergies and interactions are listed here.

Below is the Internal Item Description as shown in-game.
Note that some stat changes are not displayed here. Please see the main description for the full effects.

The Swarm

• Grants 8 orbital flies on pickup
• Each fly can block 1 enemy projectile, and transforms into a blue fly on death
• Every cleared room grants 1 new fly, up to the maximum of 16
• Abyss: x3 small shield locusts 1x Isaac's damage

• Check out the item description on the external Isaac Wiki.