Board Thread:General Discussion/@comment-26006155-20160613103837/@comment-26006155-20160614135711

Starscream1998 wrote: It's like I was telling the others, normally a 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 if Flowey only had the human souls. You need a human and a monster soul to even try and attempt anything with the barrier.

I believe Swankybox did a video about the quandry of what happens in a Neutral Undertale ending.

Out of all of Undertale's mirad possibilities, there are no eventualities where Frisk actually absorbs a monster soul. He always watches Toriel's explode, and Flowey does in Asgore's. Which means at the end of the game, Frisk is also unable to go through the barrier.

This is why I believe Frisk goes into the True Lab, and takes a portal to the past, to try to help himself out earlier to get a better ending.