Undertale - A Different Genocide Ending - Chapter 1: Frisk

What if, in the Genocide play through, Frisk didn't completely kill one boss monster? What if, they just left the Underground to create havoc without realising they were still just level 19... That boss monster will arise from the shadows and find a way to defeat Frisk once and for all...

*Side Note*
Yes, there will be inaccuracies but it's just a fanfic! Don't worry!