10 Video Games With Unimaginably Complicated Lore
3. Mass Effect
Mass Effect makes it onto this list because of the obscene amount of detail that goes into creating this version of the future Milky Way, and the way every planet, group, and historical event of note is kept for the enquiring player in the game's Codex. An encyclopedia that was added to throughout the game, even those of us who are still sore that we couldn't mark all as read within it were impressed by how much content there was.
It's the way that Bioware wove these details into the game that makes the lore of the Codex so interesting. Players didn't need to read a thing to realize that the four-testicled Krogan were calling someone brave when they said they had "a quad" because it just made sense.
Other times, the significance of Codex entries wasn't truly seen until later games, such as the planet Klendagon from Mass Effect. This planet contains a great rift that the Codex describes as a glancing blow from a mass-accelerated weapon. In the second game, The Illusive Man tracks that rift back to find the Derelict Reaper hit by that weapon 37 million years ago, and sends Shepard on a mission there.