Embryo

There are seeds called embryos that exist in some women that belong to supernatural creatures. They are usually dormant and people go about their entire lives not knowing about it. But a strong shock will cause the embryo to burst out of control, summoning it and causing havoc. Narusawa Yu is able to use his powers to subdue and remove the mutated embryo. His little sister, Mikoto, harnesses the embryo inside her to materialize a sword to fight against the summoned creatures and also keeps her perverted brother in check. With the recent outbreak of embryos at school, the siblings suspect that someone on campus is intentionally causing them to mutate!

(Source: MU)