Isaac (Boss)

This article is about the boss. For the character, see Isaac.

Isaac is the boss of the Cathedral, which can be accessed by either killing Mom's Heart 11 times and proceeding to kill It Lives, or entering an Angel Room after killing Mom's Heart and walking into the beam of light that appears. It is also accessible by using a teleport item before entering the boss room on the final floor. He is the final boss of Ending 13.

Behavior

Note: During all three of Isaac's phases, he will always shoot multiple Projectiles in the player's direction at fixed intervals. After taking enough damage, he will begin to shoot double projectiles and triple projectiles.

Isaac will not begin attacking the player until he takes damage.

Phase 1

Shoots 10 lines of projectiles in every direction.
Shoots 8 groups of projectiles in a spinning pattern.
Shoots 8 groups of spinning projectiles along with 10 slightly curved lines of projectiles in every direction.

Phase 2

Shoots 4 projectiles in the cardinal or diagonal directions, which will split into four short-ranged projectiles after a short time, similar to Cricket's Body.
Shoots 8 projectiles in all directions.
Summons Angelic Babies in pairs.
Summons two beams of light, similar to the beams summoned by Crack the Sky.

Phase 3

Isaac grows angel wings and teleports off-screen and summons beams of light to cover the screen, starting from one of the top corners to the opposite bottom corner. After teleporting back on-screen, he will shoot the 10 lines of projectiles from the first phase. He will repeat this process every time he takes a certain amount of damage
Angelic Babies cease spawning.
Uses all shot patterns from the first phase.
Isaac will block the player by flying in front of them if they attempt to traverse behind or to the left/right of Isaac.

Gallery

Boss Isaac1.png

First phase of Isaac.
Second phase of Isaac.
Third phase of Isaac.
Isaac as one of Delirium's forms.

Unlockable Items

The Polaroid - Defeat Isaac 5 times.

BRAINS! - Defeat Isaac 5 times.

PONG - Defeat Isaac 5 times.

Something Icky - Defeat Isaac 10 times.

Isaac's Tears - Defeat Isaac as Isaac.

A Cross - Defeat Isaac as Magdalene.

A Bag of Pennies - Defeat Isaac as Cain.

PAY TO PLAY - Defeat Isaac as Cain and destroy 10 Tinted Rocks.

Guillotine - Defeat Isaac as Judas.

The D6 - Defeat Isaac as ???.

Eve's Bird Foot - Defeat Isaac as Eve.

Bloody Lust - Defeat Isaac as Samson.

Satanic Bible - Defeat Isaac as Azazel.

Lazarus' Rags - Defeat Isaac as Lazarus.

Blank Card - Defeat Isaac as Eden.

Isaac's Heart - Defeat Isaac as The Lost.

Rune Bag - Defeat Isaac as Lilith.

Keeper holds Wooden Nickel - Defeat Isaac as Keeper.

Locust of Wrath - Defeat Isaac as Apollyon.

Slipped Rib - Defeat Isaac as The Forgotten.

Godhead - Earn all 5 / 9 (except Ultra Greed) / 10 / 12 Completion Marks on Hard mode as The Lost.

Also, beating Isaac with a particular character for the first time will unlock the Cross completion mark on that character's post-it note.

Strategy

Since Isaac does not attack the player until he is damaged, an effective strategy can be to use Remote Detonator after placing down many bombs. The bombs will not explode until remote detonator is used, dealing massive damage to Isaac. However, this requires the player to have a decent number of bombs (or a Golden Bomb). Having bomb synergy items (i.e. Mr. Mega, Bob's Curse, etc.) improves this strategy further.

During the second phase of the fight, attempt to damage Isaac enough to push him into his third phase as quickly as possible. The beams of lights spawned during this phase are random; the more time spent on this phase, the greater the likelihood of getting hit by them.

In Isaac's third phase, the beams of light he summons are hard to dodge, but they do have areas where damage can be greatly minimized. For example, if the beams of light start in the upper left corner, they will never hit the bottom left corner or the top right corner, but they can hit the bottom right corner. As a result, making it to a corner adjacent to where the beams begin is the optimal strategy to reduce the likelihood of taking damage from this attack, and going to the opposite corner is the next best thing as beams only sometimes hit that area. Barring these two strategies, running through the beams in the opposite direction they are coming from (if timed correctly) does not hurt Isaac.

Notes

Every time Isaac is defeated, the current savefile's image changes until The Polaroid is unlocked.

Trivia

Isaac was added in the Wrath of the Lamb DLC for the original The Binding of Isaac.
The music that plays during this boss fight is titled Infanticide.^[1] The term infanticide refers to the act of killing an infant or newborn.
Isaac, along with ???, are the only bosses who do not "die" upon losing all their health. Instead of exploding into a pile of gore, Isaac teleports away and the battle ends. Oddly it's noted that Isaac's expression as the fight goes on goes from a somber, scared tone to him smiling in his third phase. This supports the theory that Isaac has made peace with himself and realized what he must do, as seen through the multiple endings of the game.
During his first phase, Isaac makes the same crying sound as Mulligans. This also applies to ???.