A parallel world, sister to equestria, Antethica. This world is beginning it's end, degrading to nothing more than a wasteland due to the Beast. No pony has a name for it, no time when it is massive enough to devour the sun everypony held so dear. As the few ponies who held enough courage to think to resist scurried around, they angered the Beast, this malevolent creature spat a parasitic seed at Antethica. This seed grew, emitting a Sable mist that corrupted those whom took it into themselves. Some were twisted into horrific beasts, while others suffered degradation of the mind, while only going through slight physical changes. Eventually even the planet itself began to warp. The skies became a dark grey during the brightest of days, fissures appeared in the ground, from which Antethica bled a blood like liquid, and bodies of water turned to such form that it should crystallize around lifeforms whom enter, shredding them to bits.
Princess Nocturne noticed a deep and disturbing change in her sister, Princess Divinity. Perhaps she had turned, and maybe it was the loss of her sun. Either way, both changed. Princess Divinity went mad, became cruel and ill-tempered and was therefore know as Queen Nova-Storm. T
Princess Nocturnia went into a deep eternal greiving, giving little care to any pony, for all was lost. Now known as princess Moonlock.
However, some ponies, while transformed by the mist, resisted the fate of their beloved home or new world, and vowed to save what little was left. They were led by a mysterious voice in their minds, a voice that called itself Grey Heart. His voice has led a few ponies to a labyrinthine underground facility in an empty town. You hear his voice, but you are not from this world. Maybe from one of the other many realms, how did you get here. He has yet to tell you why, but it is up to you to decide whether to embrace the mists influence, or save this world from the Beast. You must decide soon, for the anguished screams of the twisted are becoming ever increasingly clear.
(https://mlpforums.com/topic/148307-omnipotence-moonlocks-heartache-ooc/)