Complex sentence: Mario uses their power to battle Super Dimentio, who upon defeat splits back into Luigi, the Chaos Heart, and Dimentio who dies before his lasting influence on the Chaos Heart causes The Void to expand uncontrollably.
Split into simpler sentences: Mario uses their power to battle Super Dimentio, who upon defeat splits back into Luigi, the Chaos Heart, and Dimentio. Dimentio dies, but his lasting influence on the Chaos Heart causes The Void to expand uncontrollably.