I would rather trust you than simply continue to spoil from within.

Church of the PlagueGrace, around the northeast direction from Gowry's ShackLandmark. You need to light the first two braziers in Sellia, Town of SorceryLandmark, then progress up the stairs north towards the Sellia BackstreetsGrace site of grace; the first two braziers unlock the barrier. Then go up the mountain towards the Church of the PlagueGrace, and she'll be next to the site of grace. After giving her the Unalloyed Gold Needle, she will move on to Gowry's ShackLandmark. Millicent can also be found in southern Altus PlateauGrace. After Godskin Apostle is defeated, Millicent can be found near the newly visible grace at the northern end of Windmill VillageGrace. This NPC moves. This NPC can be fought. If defeated as an NPC, drops: 6,000 runes, Millicent's Prosthesis (after she moved to the Windmill VillageGrace). Millicent appears as an NPC Invader, in the southeast of the Swamp of AeoniaLandmark, near the geysers and the Commander O'Neil boss fight. As an invader she is immune to Scarlet Rot, but susceptible to Poison. If defeated as an invader, drops: Sacramental Bud x5, Golden Rune [3], Golden Rune [5], Golden Rune [10], and 316 Runes. NPC Summon Boss Fights: Magma Wyrm Makar: Available once you've given her the Unalloyed Gold Needle and she's decided to go on a journey, and tried to pay Gowry a visit. Godskin Apostle: You can summon Millicent for this boss, after giving her the Valkyrie's Prosthesis and talking to Gowry at Gowry's ShackLandmark afterwards. Draconic Tree Sentinel (Capital OutskirtsRegion): You can summon Millicent for this boss, after giving her the Valkyrie's Prosthesis, and defeating the Godskin Apostle in Windmill VillageGrace. Black Blade Kindred (Forbidden LandsGrace): Advance in her questline until you defeat Godskin Apostle in Windmill VillageGrace, and her summon sign will be available north of the Statue of Marika, on the road towards the boss. Does not appear as a summon for Commander Niall, despite her dialogue stating that she would be heading toward him.