Mehrunes Dagon can be found playing poker in Cracked Wood Cave if you visit it before reaching level three. You can challenge him to a 1v1 match of Halo 2 to complete the main questline much sooner than intended.

Joke and fake loading screens and screenshots from The Elder Scrolls IV: Oblivion, run by @ChrisOfTheDead.
General links and the actual Oblivion mod made from these jokes in my Carrd page:
Mehrunes Dagon can be found playing poker in Cracked Wood Cave if you visit it before reaching level three. You can challenge him to a 1v1 match of Halo 2 to complete the main questline much sooner than intended.