Board Thread:Questions and Answers/@comment-31067473-20170803014950/@comment-33470269-20171018195244

Well, somehow, Asriel broke the barrier ( While he got 6 humans souls + All of monsters souls ), Alphys also says if she could make monsters souls to live more time out of their body, then salvation would be closer than she thought.

Maybe that could be possible with Toriel or Asgore's soul, since they are a different kind of Monster, the BOSS MONSTERS