It Lives

It Lives! is a boss that can appear on the second floor of Chapter 4. After being unlocked, it permanently replaces Mom's Heart.

Behavior
It Lives! behaves identically to Mom's Heart, with the exceptions of different spawns for its phases:
 * In phase 1 its order is now:
 * 3 Flesh Mobile Hosts
 * Polycephalus
 * Fistula
 * 2 Eyes
 * In phase 2 its order is now:
 * 2 Knights
 * 4 Walking Boils
 * Teratoma
 * Chub
 * In phase 3 its order is now:
 * 3 Lumps
 * 3 Globins
 * 4 Tumors
 * 4 Hearts

Trivia

 * It Lives! appeared in the original The Binding of Isaac.
 * The music that plays during this fight is titled Ventricide. Ventricide is a portmanteau formed from the word ventricle (chamber of the heart) and the suffix -cide, which denotes an act of killing. This theme is used in other tracks, such as Matricide, Hericide, and Infanticide.
 * Ventricide could also be a junction of the prefix ventr-, which could denote "belly", "stomach", or "womb" with the suffix -cide.
 * It Lives, according to the developers, takes the shape of what Isaac looked like as a fetus.
 * It Lives makes the same sounds as Mom's Heart.
 * Like Mom's Heart, if Isaac takes damage from It Lives' attack, Mom's laugh can be heard.
 * Despite being a major boss, It Lives doesn't belong to any Ending, instead, it opens up entrances to both and the, as well as   if beaten in under 30 minutes.
 * The unlock note for It Lives states "Your futures past awaits.", referencing Time Fcuk, another game by Edmund Mcmillen.
 * During the versus screen, It Lives' enemy/boss-spawning sound can be heard. It Lives is one of seven bosses that can be heard during the versus screen, the other six being The Stain,  The Forsaken,  Big Horn, Mom,  Mom's Heart and  Mother.