Lore
The Wet Nurse is a Great One who is seemingly sustaining Mergo. The lamp atop Mergo's Loft that spawns where the Wet Nurse was fought is called the Wet Nurse's Lunarium.
Mergo's Wet Nurse is a Great One who inhabits the Nightmare of Mensis residing in a lunarium known as the Wet Nurse's Lunarium atop the larger of the two Nightmare cathedrals. There is no written or spoken lore about Mergo's Wet Nurse, and any conclusions about the entity known as Mergo's Wet Nurse must be drawn from contextual and circumstantial evidence. When the player approaches the Wet Nurse's Lunarium for the first time they are able to hear the sounds of a baby's cry emanating from the pram in the centre of the circular area. Upon approaching the pram Mergo's Wet Nurse will drop down from the sky and stoop over to cover the pram with its wings, silencing the baby's more immediate cries. With six of its eight arms, the Wet Nurse then draws what appear to be burial blades in order to attack. The Wet Nurse's appearance and reaction to the player seem like the response of a protector or caretaker for the baby that was in the cradle. Given that the Wet Nurse is the wet nurse for Mergo, and that wet nurses (“a woman employed to suckle another woman's child.”) are only a required role for taking care of a baby, we can safely assume that the baby in the cradle is Mergo and that the Wet Nurse is attempting to protect Mergo from any harm that may come to it.
The Nightmare of Mensis contains a significant amount of references to Mergo. Mergo's Attendants and Chief Attendants roam the halls just below the lunarium. The smaller attendants seem to resemble court jesters, with their almost comical appearance and mask-helmets which resemble jester's hats. The Chief Attendants with their wide girth seem to resemble maids and helper women who could attend to household chores. The Winter Lanterns, while grotesque in appearance, can be heard singing a tune out of key. This tune may be some sort of lullaby, perhaps one intended to sooth a crying baby. These creatures all seem to serve a purpose within the Nightmare; attending on Mergo. The presence of these creatures indicate that the Nightmare of Mensis is linked to Mergo, implying that the Nightmare is Mergo's domain and that the Wet Nurse is one of the creatures that inhabits that domain. Mergo then, must be a very important baby and considering the large household of servants; perhaps even one of royal birth.
A great relic, also known as the Cord of the Eye. Every
infant Great One has this precursor to the umbilical cord.
Every Great One loses its child, and then yearns for a
surrogate. This Cord granted Mensis audience with Mergo,
but resulted in the stillbirth of their brains.
Use to gain Insight and, so they say, eyes on the inside,
although no one remembers what that truly entails.
- One Third Umbilical Cord (Mergo's Wet Nurse) description
The School of Mensis, whose actions led to the Nightmare becoming accessible were attempting to contact Mergo, and in Micolash's words be granted eyes, to have eyes planted on their brains in order to cleanse them of their beastly idiocy and allow them to ascend into something more than human as Rom once did. In order to do so the School conducted a ritual. This ritual beckoned the Blood Moon closer and allowed Mensis to gain audience with Mergo. Mergo lacks physical form. The baby was never actually born, but instead it would seem that it was stillborn; physically perishing within its mother's womb. This is perhaps why Mergo lacks any kind of perceivable physical form, instead existing as a slumbering consciousness only heard through a voice; that of a crying baby. Mergo's consciousness was awoken, but it was infantile and under-developed. Like any baby who is woken abruptly from a deep sleep, Mergo's first inclination was to cry. Special baby's act as lures to other Great Ones and Mergo's harrowing cry from within the Nightmare drew the Blood Moon closer. Whatever the details of what happened next are we do not know, but the audience of Mensis with Mergo resulted in the stillbirth of their brains, ending their life, at least in human form, en masse. The ritual conducted by Mensis seems to be the cause the Hunt (an event caused by the Blood Moon's proximity) as the baby was left to cry without recourse.
Theories
The Wet Nurse is Mergo, or was created by Mergo
The origins of the Wet Nurse
Sources
PREV: Martyr Logarius
Well, guessing the author of the nonsensical last third of this page deleted my comment, but I'd rather not let it go because this is some of the worst conjecture related to soulsborne lore I've seen this side of DS2. Can't it just be deleted or filed under Theories?
Have moved it to a separate theory - not sure who deleted your comment, but it was a good comment to make. Apologies, I tend to get a little crazy with my speculations ;).