She turns into a Mors, and is the first such creature that the party has to put down. Mors are what the fae turn into when they die via losing their purpose, and are basically living curses, incarnations of the curse on all the descendants of the six A-Rays (High Faeries, basically) responsible for the whole Lostbelt mess that never repented for their sin. The change was forestalled temporarily by Artoria Caster giving her the new name Hope after she forgot her original one, but it couldn't be put off forever because it couldn't replace her lost purpose.