Please answer a question about the following article about The Legend of Zelda: Twilight Princess:

The game features nine dungeons—large, contained areas where Link battles enemies, collects items, and solves puzzles. Link navigates these dungeons and fights a boss at the end in order to obtain an item or otherwise advance the plot. The dungeons are connected by a large overworld, across which Link can travel on foot; on his horse, Epona; or by teleporting.

Who must Link fight at the end of a dungeon level?
boss