Equestria watched fearfully as the Crystal Empire fell from an unknown magical disease, or more commonly said; curse, turning ponies into undead, crystalline zombies. Later it was called Sombra's Light. Nopony knows how it came to be nor the ingredients to the cure. The Ministry of Defense tried their very best to quarantine the Crystal Empire, but the curse spread all over Equestria, causing mass hysteria.
Some questionable actions had been made, leading to the utter destruction of Van Hoover, and leaving the world shocked. Celestia has refrained from ever using the Sun's power again, wishing her little ponies a heartfelt good luck, before going into hiding. Accounts on the whereabouts on Luna range from turned into one of the Cursed to being trapped on the moon again. The disappearances of the two princesses, angered the population, which led to the ransacking of Canterlot, making the Capitol a deathtrap.
The Equestrian Relief Effort (ERE) set up hospitals and witch huts all over, trying to stem the Light from cursing more ponies. Other creatures have fled the country, despite the Light proving to be completely uninterested in them. The danger of the Cursed is still a threat to them. Over the years, ERE activity has lessened as the Light keeps on getting more and more out of control. Before their demise, however, they discovered that specialized crystals combined with intense light, repel the creatures of the night.
Each city, with their mayors, had a different way to prevent or stall the Light outbreak. Fillydelphia, one of said cities, was outfitted with a low gravity field encompassing the entirety of the city, with the mayor hoping that it wouldn't spread as easily. Survivors of Filly have banded together and created various factions over the years to survive. Every month, Battlekour competitions are held, the winning faction claims ownership of Filly until the next competition.