Millicent

Solve the Sellia puzzle to continue up the hill to the Church of the PlagueGrace. Speak to Millicent and give her the needle. Reload the area and speak to her again after she's recovered to get the Prosthesis-Wearer Heirloom

Find Millicent in Gowry's shack and speak to her.

Retrieve the Valkyrie's Prosthesis from a chest on top of the Shaded Castle in north Altus.

Bring the Valkyrie's Prosthesis to Millicent at the Erdtree-Gazing HillGrace Grace.

Defeat the Godskin Apostle in Dominula, Windmill village. Reload the area and speak to her near the Grace.

(Optional) Summon Millicent to aid you against the Draconic Tree Sentinel.

Find Millicent again at the Ancient Snow Valley RuinsGrace Grace in the Mountaintops of the GiantsRegion

Meet with her once more at the Prayer RoomGrace Grace in the Haligtree

Defeat the Putrid Tree Spirit inside the tree.

Reload the area and use the “help Millicent” or “challenge Millicent” summon sign (we recommend help). Reload the area again and speak to Millicent. Be very careful not to jump down and land on top of her, which will kill her and fail the quest.

Take Millicent's needle to Malenia's arena after defeating Malenia and use it on the flower to acquire Miquella's Needle and a Somber Ancient Dragon Smithing Stone.

(Optional) Use Miquella's Needle in Dragonlord Placidusax's arena to remove the Flame of Frenzy.