Akatosh is one of the Aedra, or Divines. He is the god of time and the chief deity of the Nine Divines. He is depicted as a dragon, and chose that form to battle Mehrunes Dagon in the Oblivion Crisis.
In The Elder Scrolls IV: Oblivion, Martin Septim smashed the Amulet of Kings and took on the form of Akatosh to battle Mehrunes Dagon to save Tamriel and the Imperial City from the encroaching Oblivion invasion forces, killing himself in the process. After Mehrunes Dagon was defeated, Martin turned to stone whilst still in the form of Akatosh, and now stands as a statue in the temple as a reminder his sacrifice.