Tardis11Gal
Frisk and Asriel are now the rulers of the surface with the envious Chara riding their coattails, successors of their three murdered parents...Sans, Toriel, and Asgore. 20 years before the events of the story was when the barrier was shattered, the monsters were freed, and the Underground was evacuated. The humans went on high alert, causing Undyne, Papyrus, Sans, Asgore, Mettaton, Toriel, and the newly restored Gaster (Gaster sub-plot slightly changed as of now) to have to wage war on the surface. Long story short, Sans, Asgore, Mettaton, and Toriel were killed. Chara, Frisk, and Asriel used the DETERMINATION caused by their parents' deaths to fight back...they won.
20 years after the war, Frisk is the Queen of the Surface with King Asriel as her husband.
Read what happens when the humans rebel and try to take back their rule of the surface.