To get here, you must be killed by a Kidnapper. Kidnappers start appearing after killing Blood-starved Beast (in Old Yharnam), and one is at the bottom of the straight stairs in Cathedral Ward.
Hypogean Gaol is an optionally accessible part of an end-game area called Yahar'gul, Unseen Village. You only have access to a small part of it (and with less dangerous enemies), until Blood Moon. It can be worth visiting: the Kidnappers near the lamp drop Twin Shards and 1500 blood echoes each, and the one-directional shortcut doors you can now open will let you exit the church safely during Blood Moon. Also, encountering Darkbeast Paarl turns Djura back in Old Yharnam non-hostile, letting you talk to him.
Indoors
Lower cells
The area where you wake up, including the downstairs.
Note ("Madmen toil")
Just outside the cell that you wake up in.
You must be wearing the chest piece of any church clothing (including Gascoigne's) to speak to her.
Only possible before Blood Moon.
Send her to
Sending here to the clinic will prevent acquiring "Church Bow (Female)" gesture.
Chapel
Go all the way up the circular stairs from the basement.
Hypogean Gaol
Note ("Nightmarish rituals")
Next to the lamp.
On the lower circling staircase, on the right (facing the church from the lamp).
Moon (+10%)
Street
Note ("Behold!")
At the top of the large stairs that the Maneater Boar patrols.
Exit the church, turn left and go along the wall towards the back of the church.
Church back door
In the lowered alcove off the side of the street.
Gate
Upper cells back entrance
Exit the church through the front door, turn right and go along the wall to the back of the church.
Door to the upper cells
In the upper cells under the chapel, go through the hole in the wall in the back.
Encountering Paarl lets you talk to Djura back in Old Yharnam, see Djura (non-hostile) for details.
Graveyard of the Darkbeast