What Are Souls of the Dead?

S1 E6: Tatebayashi and Senge (in cute form) square off in one of the Imperial Army's meeting rooms. Senge sees that Tatebayashi is struggling, and says he'll go after Kyoichiro himself. Ignorant of all this, Kyoichiro is returning home at evening, when he stumbles upon a few soldiers recovering a soul of the dead.