First, the player needs to locate at least three umbilical cord segments. One can be looted from Mergo’s corpse. Another can be found in the Abandoned Old Workshop. The third can be looted from Arianna’s monstrous newborn after killing it. A fourth can be found on Iosefka after killing Rom the Vacuous Spider.
Return to Gehrman and refuse his offer, which triggers his boss fight. Make sure to consume three Umbilical Cord segments before Gerhman is dead. They can be consumed at any point in the game, up to and including his boss fight. Consuming the Cords and defeating Gehrman will unlock the secret “Childhood’s Beginning” ending.
“Childhood’s Beginning” proceeds like “Honoring Wishes,” but instead of submitting to the mysterious entity, the player battles it. The entity is revealed to be the Moon Presence, a tentacled Great One. The Great Ones are Bloodborne‘s demonic gods, inspired by the stories of H.P. Lovecraft. The Moon Presence is the most powerful and has created the Nightmare that plagues Yharnam.
After defeating the Moon Presence, the Hunter is reborn as a slug-like Great One themselves, giving a whole new meaning to the phrase “God Mode.”