Long, long ago, royal scientiest W.D. Gaster was the most highly respected scientist in the entire
One day, however, he ended up falling into the core for an unknown reason. This is where it gets interesting.
His body ends up dealing damage to the CORE and overheating it on impact. The core begins having problems, before finally straight up exploding. The explosion toxicates the whole underground and devestates it completely. Snowdin is nothing but a dead wasteland full of burned trees, ashes and dirt now.
Without the warmth of the CORE, Waterfall froze and now is nothing but an ice cave. Because of the lack of a heater, Hotlands magma also turned into vulcanic stone, leaving the Hotlands to cool down. With the CORE gone and no power being given to the UNDERGROUND, it turned dark really quickly. However, nature finds its way.
The radiation that the UNDERGROUND was exposed to caused mutations in flora and fauna. Many Monsters soon began to notice that some parts of them appear to be glowing in the dark in a light shade of blue. Plants also seem to adapt this feature, as glowing blue mushrooms soon begin to grow all around the UNDERGROUND. Life soon continues, but there sure is a lot of oddities down there now...
. . .
Then you notice something glowing on yourself...
Base from WEZ (deviantart) and edited to show shadowtale situation.