Complex sentence: Dimentio takes control of Luigi again, as the Dark Prognosticus revealed him as the ideal host for the Chaos Heart, and fuses the Chaos Heart, Luigi, and himself into one entity called Super Dimentio, who has complete control over The Void.
Split into simpler sentences: Dimentio takes control of Luigi again, as the Dark Prognosticus revealed him as the ideal host for the Chaos Heart. Dimentio fuses the Chaos Heart, Luigi, and himself into one entity called Super Dimentio, who has complete control over The Void.