This is the true ending to Bloodborne. You first need to get all 3 pieces of the "One Third of Umbilical Cord". Once you have them, you want to consume them all by selecting "Use" on them in your inventory. Doing so will give you some insight, but they are needed for the secret ending. Spoilers are ahead. Proceed at your own risk.
Here's a video guide of how to get the all of the One Third of Umbilical Cords.
The first one you can’t miss, it drops off the Wet Nurse boss.
For the second, you'll need access to the Abandoned Old Workshop. Which can be accessed from the Cathedral Ward area.
For the third, you'll need access to the top of Iosefka's Clinic. Here's a video to get you there. This one is missable.
There is a fourth One Third of Umbilical Cord. But you only need three for the trophy.