Talk:Asriel Dreemurr/@comment-27189387-20151208155754/@comment-1365839-20151208160342

Because he needed power equivalent to 7 human souls to regain his true form. He absorbs the 6 human souls, but only in the True Pacifist route he's able to also absorb the souls of almost all the monsters in the Underground, whose power equals more or less to another human soul.

tl;dr: He needed 7 souls. Sum of all monsters' souls = 1 human soul