Board Thread:Questions and Answers/@comment-37807741-20190111181155/@comment-38040690-20190111221719

The monsters left the underground without a human soul because, in True Pacifist Route, Asriel breaks the barrier after you spare him. Remember, he has absorbed the 6 human SOULs, and the SOUL of every monster in the underground, which is equal to seven human SOULs, which allows him to break the barrier.