Complex sentence: For example, the Dungeon is inaccessible before Skeletron's defeat, and is unaffected by the activation of Hardmode until Plantera is defeated, upon which the difficulty of Dungeon enemies rises drastically with new enemies.
Split into simpler sentences: For example, the Dungeon is inaccessible before Skeletron's defeat, and it is unaffected by the activation of Hardmode until Plantera is defeated. Upon defeating Plantera, the difficulty of Dungeon enemies rises drastically with new enemies.