Lost Soul is an unlockable familiar in The Binding of Isaac, added in the Repentance DLC.

Spawns a familiar that follows closely behind Isaac and dies in one hit, respawning at the start of the next floor.

If the familiar survives and is taken down to a new floor, it will rewards Isaac with one of the following:
• 3 Soul Hearts.
• 2 Eternal Hearts.
• 1 item from the Treasure Room or Angel Room item pool.

Has a 1% chance of being dropped upon defeating Blighted Ovum.

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 Mother as The Lost.

Lost Soul can be found in the following item pools:

The following tags are applied to this item:

The following special synergies and interactions for Lost Soul 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.

Lost Soul

• Following familiar that dies instantly upon taking a hit
• If the familiar survives for an entire floor, it will grant 3 soul hearts, 2 eternal hearts, or 1 random angel room pedestal item at the start of the next floor
• If killed, the familiar regenerates at the start of the next floor
• Abyss: x1 locust .75x Isaac's damage

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