Monster Cells

Monster Cells (怪人細胞, Kaijin Saibō) are cells that can transform Humans into Mysterious Beings.

Overview
It has the appearance of a pulsing sludge, that if humans would inject those cells into themselves, they would supposedly start to exceed the limits of human beings and turn into monsters.

Monster Association Arc
Haragiri used Monster Cells to turn into a Mysterious Being during the meeting of the Council of Swordmasters.