The Executioner and Her Way of Life, Vol. 4 (manga) — Mato Sato

In the port city of Libelle, where one of the Four Major Human Errors is sealed, Menou and company are tasked with finding the origin of the drug monstrine. They each begin their undercover operations, but nothing goes quite according to plan -- Menou is attacked by an unexpected foe, Momo is caught in a trap, and Akari goes missing! And when the time comes to face off against their greatest suspect, Manon Libelle of the Fourth, Menou will learn just how terrifying a Human Error truly is...