You were a yiga clan member and it was 100 years after the calamity, and now there was an upheaval. You were snooping around the great plateau for nostalgia when you fell down a hole with a gross red haze surrounding it.

after falling for a good amount of time, you landed in some sort of really dark place, with a mine in it

you walk for a bit before stumbling upon some sort of structure, and standing there seemingly struggling with some sort of Zonai device, was Master Kohga. The leader of the yiga clan that everyone thought link killed off.