Isaac can encounter It Lives in the nightmare boss wave of chapter 4 in Greed/Greedier mode even if it has not been unlocked.
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.
According to Edmund McMillen, It Lives supposedly represents Isaac wishing that he were never born.
It Lives makes the same sounds as Mom's Heart.
Like Mom's Heart, if Isaac takes damage from It Lives' Brimstone 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 Sheol and the Cathedral, 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.
Sometimes, if the boss intro is skipped, the game will not teleport Isaac into the center of the bottom of the room, resulting in immediate contact damage with one of the Eyes or It Lives, depending on the direction of the room's entrance.