From Halopedia, the Halo wiki
.fortune is an Easter egg in the Xbox version of Halo: Combat Evolved, as well as Halo 3, Halo: Reach, and Halo: The Master Chief Collection. In all cases, the egg causes a funny message to appear when ".fortune" is written somewhere, but the egg is more extensive in Halo: Combat Evolved. In Halo 3 and Halo: Reach, giving a map variant the description of ".fortune" will produce the phrase "My modem is on fire". In Halo: Combat Evolved, if the player creates a profile, renames an existing one, or creates a map name and names it ".fortune", a random fortune-cookie-like saying will appear on the screen. In The Master Chief Collection the Easter egg will appear if the player enters the .fortune almost anywhere in the game, including map variants, game types, loadouts, clan tags, or even Service ID.
The ".fortune" on the text box will then also instantly disappear. However, if the player places a symbol at the beginning (such as a left parenthesis) to make ".(fortune", and subsequently uses the arrows to move to and delete the parenthesis when finished, ".fortune" will remain, allowing the player to use it as a profile name if they so wish.
Activating the Easter egg in Halo: The Master Chief Collection unlocks the Mystery Achievement.
List of Fortune Quotes in Halo: Combat Evolved