Lazarus
ReadyHave 4 or more soul hearts at the same time.
Achievement
Defeat Mom's Heart 11 times and unlock Lazarus.
In-game: “Unlocked challenge #7.”
Have 4 or more soul hearts at the same time.
Defeat Mom.
Reach Depths II and defeat Mom.
Sources:AchievementsWomb
Defeat Mom's Heart 11 times.
After: The Womb
Defeat Mom's Heart 11 times and unlock Lazarus.
After: It Lives!, Lazarus