Who Would Win?

No idea

Not really how the template works, just sayin'.

the entirety of undertale would also be all the human and monster souls, if all of them fuse into 1, hollow knight is beyond cooked

*ahem* radiance, ABSOLUTE RADIANCE, the pale king as a wyrm, not to mention the 50-60 gods the player oh so casually kills. Also SHADE LORD.

with only 8 human souls one becomes god, in the 2010's there were about 7 billion people on earth, just 10 souls alone cooks any god in existance

What about the 57 or so in godhome just chilling

7 billion human souls could probably evicerate multiverses. Nothing can survive that.

The shade lord terminated all of godhome and the lands beyond hallownest with ease. Humankind stands no chance .