Sorry for the clumsy English — I use a translator

The alternate history of MLP G4 is when the Stygian Darkness (season 7 — the last two episodes) gets out of control and becomes the most deadly and dangerous evil that no one can stop. At first, scientists discover the Hall of Darkness, from where everything will begin. After releasing the darkness, it concentrates into a gas, starting to absorb air, light and hope. The first ponies can survive, if they try running away. Or they become supporters of darkness — of course, losing their will and their "Humanity" (conceptuality), becoming puppets in the hands of Darkness. Then the Darkness begins to spread throughout Equestria — to zebras, griffins and ponies.

Subsequently, the Darkness becomes so strong that ponies who have touched the darkness begin to “evaporate" as in the Avengers (or "Raya and the Last Dragon")
Slowly — if the pony is strong in spirit and magic, or quickly or even instantly if the pony is “not so kind in his soul" or very afraid.
Darkness is gathering, filling the streets of the city. The guards of the princesses, who gave their lives and were later named the Light Hundred, create light shields, delaying the coming darkness so that simple ponies can escape. Some guards can't stand the pressure of Darkness, or even get so scared of it that they lose concentration with their magic and the Darkness kill them. There is no vaccine. As well as salvation. Only Darkness!

The idea came from music: Radiarc — The Immortal Void (release of "Pony at Dawn")