The massive side-story starring Ada Wong . This was originally a PlayStation 2 exclusive that explains what Ada was doing during Leon's main mission.
If you are looking for the most complete version of the game, the Resident Evil 4 Gold Edition is the current standard, bundling the base game with all major expansions.
: Lore hunters often compare the game's cult documents to the real-world Codex Gigas (The Devil's Bible), analyzing how the Los Illuminados faith is modeled after medieval occultism.