When a mysterious cult of ponies kidnaps Button Mash and takes him to a mysterious island to be sacrificed, the Mane Six, along with four stowaways, set out to save him and find out why he of all ponies was targeted. However, the reasons behind this may bigger, stranger, and more malicious than anything the ponies could ever imagine. The Dark Entity is asserting its dominance over their world once again.
(The title is clearly a reference to "Hades Persistence," the title of an episode to Beyblade Metal Fury.)