I think Mergo's Wet nurse is Mergo himself. First of all, she's a has no shape (as Mergo, who we can only hear, while he's crying). Mergo has "created" the wet nurse because he's only an infant and he can't survive alone. This is the reason because (it seems to me) the wet nurse is the only Great One, who is weak to fire, bolt, poison, and also arcane; because she's only a "personification" of Mergo. So I think that when you kill her, you kill automatically Mergo; in fact the baby stops crying.

If she still glows purple even after her nightmare fog "disappears", do NOT let your guard down. Her bs clone will still be floating around for the rest of the fight. I can only assume this is a glitch or something. I've soloed her 4 times and this has never occurred to me until I started helping others as a summons.

There's a tip above about killing the clone and then being free to harass the boss while the purple fog phase lasts. Well I just killed the clone straight away, another one appeared soon after. I killed that one, another one appeared. Don't know if the game was glitching out for that guy, maybe the boss was in "stupid mode" because of the memory leak glitch the game used to have.

Vaatividya suggested that this thing might have some connection to Formless Oedon, given the lack of body. Plus, as Great Ones need a surrogate it stands to reason that Queen Yharnam was said surrogate - heck, she's practically been raised for it according to the Ring of Betrothal (and the various Vileblood lore bits). Or at the least the nurse could be a servant of Oedon, much like the 'Shadows of Yharnam' that wander the area...

I killed mergo with the chikage. It might drain your hp when transformed but does a great damage to her. Try it!!

Why does Mergo not have his/her own page?

Has anyone else noticed that's the corpse during the orphan of kos fight and mergo are very similar in build and design? Besides phantom appearance and fish shapes they could be the same model or more closely related

I'm on my first play through. Followed most of these instructions, but when it started getting dark, I just kept attacking and killed her before the clone had a chance to spawn (first Nightmare Veil). I was using a +10 Saw Cleaver with bolt paper. 38 strength and 25 skill.

I killed her super quick. She only ever used her flurry attack while walking in a big circle. All I had to do was walk behind her and whale on her with the Phantasm Shell applied. Only used two blood vials the entire match. Didn't see any other attacks, or a clone.