This achievement is for beating The Womb for the first time. The Womb must first be unlocked by killing Mom once. After that, every game from then on will progress to The Womb instead of ending at Mom’s death.
If you’re having trouble getting this achievement, I suggest using Azazel. A guide to unlock him is:
*** Spoiler - click to reveal ***to make three deals with the devil in one game. To do this, you need to find devil rooms. These rooms spawn after you kill a boss, but they are random and may not always appear. To increase the likelyhood of one appearing, you need to not take any red heart damage during the boss fight.