Most of the Nightmare Apostles hung out in Mergo's Loft, but who or what is Mergo?! Actually never seen, Mergo is a little baby Great One believed to be the offspring of another unseen Great One, Oedon, and the queen of the Pthumerians, who shares her name with Yharnam. But while we never see what their child looks like, we do battle what's apparently its caretaker, its wet nurse, traditionally referring to a woman who takes care of someone else's baby and breastfeeds them for their biological mother, if she happens to be unavailable or even dead.

Mergo seems to be sort of humanoid, albeit very very tall and sinuous, with six long, shriveled grey-black arms that wield bladed weapons and a set of black-feathered wings. She's also entirely draped in many layers of flimsy black fabric, including up the back of her head and neck, though her actual head and neck seems to be totally invisible! This results in the unique aesthetic of a reaper-like figure whose empty hood is long and serpentine in shape!

A major battle gimmick of the nurse is one of the oldest cheap tricks in the boss monster book; she spawns false copies of herself that are easy to destroy, but come right back and can hurt you just as bad as the real thing. I'd admonish her for how rude this behavior is, but I can't fault a single woman in a thankless job for doing whatever makes that job easier.

Overall I love the effect of fighting not the Great One itself, but "just" its "wet nurse," who is already so deadly and so monstrous anyway. And what does it even mean to be Mergo's Wet Nurse? Is Mergo even a thing that would have drank milk, or had any other needs comparable to a human infant? After defeating her, you'll receive one of three umbilical cord items, rare artifacts left over from the birth of great ones and capable of providing lots of juicy insight at once. Nothing like a monster's umbilicus to get your brain-eyes popping!

Ominously, every umbilical cord says "Every Great One loses its child, and then yearns for a surrogate." EVERY Great One? Their babies always die?! We'll see eventually that no, not quite, but whether they're better off will be the real question.