You can summon her for the Magma Wyrm Makar fight, although she really only is there to make the fight harder lmao

After giving her the needle and reloading you get the talisman. Afterward if you go to the shack where the old man was, he's gone but now millicent is there and has extra dialogue, perhaps a quest line.

After giving her the needle, reloading, she will give you a talisman, Visit Gowry's shack and she will be there and he will not, if you visit again after a reload, she will be gone and he will have arrived and you can now learn spells from him.
You can then find her at the Erdtree-Gazing Hill, where she says she wishes she can aid you but she has no arm (which is strange since you can summon her in the Magma Wyrm Makar fight before this interaction)
However you can go to "The Shaded Castle" and in a chest guarded by a difficult enemy there is a "Valkyrie Prosthetic" which can be given to her back at the Erdtree-Gazing Hill Grace, which will cause her to leave the area again.

Uh. I didn't clip it because I thought she was just another character. But she invades you outside O'Neil. I'm not sure the requirements but the sword she wields practically let's her fly at you. Pre killing boss.

Very intruiged to see where she goes plot wise, SPOILERS (kinda) later on you can give her a prosthetic valkery arm. She has red hair. Memory loss. And a fake arm? Reminds me of someone in the trailer.

She invaded me right after the dialog with Gowry in the swamp near Commander O'Neil, I still haven't met her as an NPC

Does anyone know where she goes after giving her the prosthetic arm?

She starts out at the church of the plague, moves to Gowry's shack once you give her the gold needle and exhaust dialogue, will move to the Erd tree gazing hill after that, and then to Dominula the windmill village after defeating the boss there.

You get the unalloyed golden needle by killing Commander O'Neill in the center of the swamp.

It's above ground.