Mergo is a mentioned character in Bloodborne.
Description[]
Mergo is an infant Great One, suspected to be the child of Yharnam, Pthumerian Queen and Oedon, a Great One.
The child appears to have been stillborn in some fashion, as evidenced by the item description of a certain Third 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." Mergo is all but stated to be the cause of the Nightmare of Mensis.
A pram implied to be housing the infant unborn Mergo is found in the Wet Nurse's Lunarium located in Mergo's Loft Middle, guarded by fellow Great One Mergo's Wet Nurse, whose title implies she was appointed as Mergo's caretaker.