Calamity Ganon

Calamity Ganon is the "main" or "final" boss in The Legend of Zelda: Breath of the Wild, although he can be fought at any point in the game.

Description
Calamity Ganon is the main enemy that must be fought. He is located in Hyrule Castle, and is the final boss. The outside of Hyrule Castle shows a Black with Red aura spinning around the perimeter, but it is revealed that it is actually a giant creature with many weapons and attacks that makes it nearly impossible to kill.