The Elder Scrolls IV: Oblivion

Oblivion is a single-player game that takes place in Tamriel's capital province, Cyrodiil. You are given the task of finding the hidden heir to a throne that sits empty, the previous emperor having been killed by an unknown assassin. With no true Emperor, the gates to Oblivion (the equivalent of hell in the world of Tamriel) open, and demons begin to invade Cyrodiil and attack its people and towns.