When you bring the faerie hearts in Tainted Grail: The Fall of Avalon, Phaidros will ask you to find the brains of those mutated by the Red Plague. In this guide, we’ll tell you where to find the monsters that drop these rare ingredients.
Give the ingredients to Phaidros during the "Fragments of Humanity" quest, after which Alma will ask you to bring the brains of: a Nuckelavee, a Marrow-Touched Peasant, and a Red Death Infected. Collect the components and bring them to the girl, after which you will receive 3,300 experience points.