Iron Heart is a mechanics in Benighted Soul, a mod for The Binding of Isaac.

Protects Isaac from taking non-self damage.
Loss and invincible time are related to the damage type.
(Detail in GuruWiki)

Cannot prevent Isaac from dying.

Releases shockwaves when cleared by hurt.

To other characters:
• Will be cleared next level.
• Fixed 28 maximum.

To Benighted Magdalene:
• Causes 1 heart extra damage when taking over-damage.
• Maximum range [14, 70].
• Tiggers hurt effects (no Hp loss) and lose 2 maximum for every 7 loss, and then releases shockwaves.
• Recovers 1 maximum when clearing a room. ^IRecovers 0.5 maximum when entering a new greed wave.
• When gained from some other effects, the part over the maximum will be saved until the next room.

Format: DamageType: Loss; InvincibleFrames.
Loss and invincible frames are stackable.
(60 invincible frames = 1 second)

• Projectile: 6; 20
• Enemy: 2; 5
• Creep: 2; 60
• Explosion: 33; 100
• Spike: 2; 60
• Laser: 1; 1
• Fire: 1; 60

• None above: 1; 3

• Download Benighted Soul from the Steam Workshop.