Replying...
Intro. This story started millennia ago when Equestria barely knew Peace, A dark entity existed, it's name was "Nocturnis" a ancestral spectre created from the worst fears of the Ponies, Nocturnis did not have a physical form, not in that moment, at least. It manifested as whispers in the dark, nightmares and shadows. It fed from Panic and despair, it's power growing because of the Fear. Like a silent plague. But one day, as he reunited enough magic, to obtain a physical form, the Princesses confronted nocturnis in a desesperate battle, they couldn't destroy it, since fear cannot be erased. But they sealed it in a rock, as time passed, nocturnis was forgotten, but he was still there...

My little pony:The whispering plague

@Eclipse