Strike the Blood, Vol. 13 (light novel): The Roses of Tartarus — Gakuto Mikumo

This is only the beginning...
Chaos descends upon Itogami Island! Air and sea transportation have come to a grinding halt and key personnel from the Gigafloat Management Corporation are being assassinated one after another. The proud culprit behind this series of attacks is the self-proclaimed Demon Sanctuary destruction group, Tartarus Lapse. At the same time peculiar changes begin to occur within Nagisa as the soul of the Avrora bleeds into her consciousness. In her time of spiritual turmoil, a mysterious girl appears before her and introduces herself as "December." What Nagisa doesn't know is that this girl is the leader of Tartarus Lapse, and she has a special ability capable of overwhelming even the Fourth Primogenitor...!