The usual "final" battle of Bloodborne is actually unlocked on defeating Mergo's Wet Nurse. If you subsequently return to the Hunter's Dream, you'll find Gherman's workshop on fire, and the doll will inform you that he's waiting for you elsewhere. Gherman will actually offer to free you from the hunt by killing your dream-form, and if you accept, you really awaken back in the "real world," sun rising over Yharnam after the blood moon has passed.

Refuse, and Gherman will attack anyway in an attempt to kill you for real. Why he does all this is another aspect still debated, some believing that he truly doesn't want to see you lost to the dream like himself, while others believe that you're becoming something so dangerous and so inhuman, his duty as a hunter forces him to put you down.

If you defeat Gherman, a strange entity will descend from the Blood Moon, and your character will put up no resistance as they're wrapped in its claws and tentacles. You'll come to in Gherman's wheelchair, tended by the Doll, having taken his place as the "host" of the Hunter's Dream.

...But who was that Great One?

The moon above Yharnam is actually not an orbital satellite, but floats within the sky only miles away, and it's either the dwelling place of a Great One or a Great One itself. Whichever the case, this being is commonly known as Moon Presence, less commonly as Flora, and can manifest a terrifying physical body when the need arises; a lithe skeletal humanoid with far, far too many crooked ribs down the long length of its spine. Its arms and legs are slightly more fleshed, as are its long, powerful, branching tails.

Flora's head is almost nothing but a twisting, writhing mass of huge, black tendrils, like an immense knot of dark worms or serpents, one of which flares into a knob that constitutes its "face," though it's hard to say if the gaping holes in it represent any nameable features. You could actually kind of describe the head as something like a giant, black sperm cell surrounded by dozens of additional tails.

This monstrous form is surprisingly a bit weaker than Gherman, though it has some seriously cheap and nasty attacks. If it kills you, you reawaken in the Hunter's Dream where you can tackle the fight again and again. What happens when you win the fight is a very different matter, and don't worry, we'll get to that.

What is known about Flora's relationship with Gherman is that sometime after Lady Maria disappeared, Gherman evidently called out out to the lunar Great One to help bring his creepy Maria Doll to life. In exchange, he was forced into a contract that allowed Flora to create the Hunter's Dream from Gherman's mind. Why it wanted all this isn't entirely clear, especially because the Hunter's Dream exists primarily to aid more hunters, whose ultimate purpose in existence is to battle against the work of the Great Ones and even slaughter the god-beings themselves.

There are, of course, innumerable fan theories filling in the blanks. Some believe that Flora has a motivation to kill fellow Great Ones and reign supreme, but that sounds a bit too on the nose, to me; too "comic book supervillain" for a Souls game.

We heard before that "Every Great One loses its child and yearns for a surrogate," and in the ending that sees you replacing Gherman, Flora lifts and cradles you in a way many players interpret as tender and motherly. It feels, perhaps, like all the being really wants is that "surrogate child," even if they're only a human kept indefinitely and unnaturally alive in the dream. A crude and empty replacement, filling a role to the Great One that ironically reflects the existence of the Plain Doll. The training of other hunters may well have been more Gherman's idea than Flora's, and perhaps another reason he kills every one of them is so they don't ascend to the status of Kin.