Board Thread:General Discussion/@comment-26849575-20160315015552/@comment-13682699-20180127010845

We need Asgore's SOUL in order to absorb it so that we obtain enough power to pass through the barrier and escape the underground.

You need a human and a monster SOUL combined to escape. As the only human, you possess a human SOUL. In order to acquire a monster SOUL, you would either need to take a SOUL from a living regular monster, which would take extreme amounts of power, or you could kill Asgore and take his SOUL since, as a boss monster, his SOUL will actually survive his death for a short period of time.