Ok, that's DeceptionTale, now on to Shadow's actual backstory! (*warning: I (Shadow irl) came up with it after listening to music, so yeah I don't know why this was a warning XD*)
I was born a green-souled human with an older orange-souled brother called Sans. Our mom was non-existent, I'm unsure whether she was dead but I never met her. Our dad was a scientist who was trying to understand soul magic to try to create human soldiers who could use magic. Nothing wrong with that, (well sort of I'm from Underfell/Undertale don't judge me) except that he experimented on me and my brother, ending up with both me and my brother having half-white souls. Dad died in the explosion (good riddance). Me and my brother ended up in an orphanage (the Papyrus of our world was there, rather than in the Underground). Those who had partially white souls were discriminated against (magic blood ran in families who had had hybrid children before the war between magic humans and normal humans and remained into my generation) and me and my brother had our rooms graffitied, our stuff stolen and shit. Normal bullying. I ended up being attacked in a car park when I was 12, basically beaten to near-death, and thrown down Mount Ebott so no one would find out. I was nursed back to health by Toriel (healing magic) and then left for the Underground after persuading Toriel. I was attacked by a Dog sentry (I ended up petting and befriending it), and it defended me from all other monsters on the way through Snowdin (I wasn't attacked in Snowdin Town as the dog had been stealing items and attacking people, and I had stopped it and thus the townspeople liked me a bit) and most of Waterfall. Until Undyne. She attacked, but before she could begin to attack she noticed my half-white soul, which wasn't strong enough to be used to break the barrier. She grudgingly released me, allowing me to spend my time as a subject of the Underground.

Next chapter: Shadow's story continued