If you refuse to submit your life to Gehrman, the Moon Presence—a Great One—comes to take you. She doesn’t come to kill you; it’s more like recruitment or indoctrination. However, if you have consumed the three Third Umbilical Cords, you become like unto a Great One yourself, and she has no power over you. This causes Moon Presence to fly into a rage and attempt to kill you. After her defeat, you are reborn as an infant Great One.
This is the ending that seems to most please the Plain Doll. Presumably she expects things to improve with the Moon Presence destroyed and the Hunter reborn. Given that, I believe this is intended to be the ‘best’ outcome for the PC. There is certainly no indication of the Doll having a sinister ulterior motive, but the possibility she is the prime architect of the Nightmare cannot be completely dismissed. Perhaps she now has greater power with the Moon Presence out of the way and she holds sway over the infant Great One in her arms.
I don’t think that’s the case. I think this is supposed to be a positive ending, but I have trouble getting past the fact that I’m now a slug.