Complex sentence: The Daedric Prince of Destruction and Deceit, Mehrunes Dagon, appears as the main antagonist in, where he attempts to expand his realm beyond the plane of Oblivion, thereby ending the world of Nirn and the land of Tamriel.
Split into simpler sentences: The Daedric Prince of Destruction and Deceit, Mehrunes Dagon, appears as the main antagonist in, where he attempts to expand his realm beyond the plane of Oblivion. Therefore ending the world of Nirn and the land of Tamriel.