Board Thread:General Discussion/@comment-26006155-20160613103837/@comment-27089028-20160614113355

It's like I was telling the others, normally I monster soul cannot absorb another monster soul except when that soul has obtained a human soul and has become something entirely different therefore the rules no longer apply.

And besides, how was Frisk able to get back to the surface I Flowey only had the human souls. You need a human and a monster soul to even try and attempt anything with the barrier.