Progress Millicent’s questline until she appears at the Ancient Snow Valley Ruins in the Mountaintops of the Giants
Defeat the Godskin Apostle in the Windmill Village if you have not already progressed far enough to unlock her later steps
Obtain the Valkyrie’s Prosthesis from the chest in the shaded room at the top of the Shaded Castle in Altus Plateau
Give the Valkyrie’s Prosthesis to Millicent at the Ancient Snow Valley Ruins
Defeat the Ulcerated Tree Spirit at the Drainage Channel in Elphael, Brace of the Haligtree
Choose to assist Millicent when her summon signs appear near the Prayer Room area
Defeat the invaders and complete the quest interaction after the fight
Receive Millicent’s Prosthesis as a reward from Millicent’s questline completion path