Board Thread:Questions and Answers/@comment-30088034-20170219194117/@comment-26887879-20170222211926

Also, they couldn't escape even if they wanted to. There weren't enough human souls, to break the barrier. Maybe they couldn't get past Asgore... (But I'm starting to believe that it was Sans who ended them all. Maybe back then he didn't know Toriel and didn't make any promise)