Quake Episode 5: Dimension of the Past

Episode 5: Dimension of the Past (called "DOPA" for short) is an add-on episode for Quake developed by MachineGames, to commemorate the game's 20th anniversary. It's available to download for free.[1]

Despite not being developed by id Software themselves, the episode is considered semi-official, being developed by a fellow Zenimax-owned company that has previously co-operated with id Software. The episode also contains many allusions and callbacks to Quake proper, such as the re-appearance of the Dopefish easter egg, along with the final level of the episode being almost identical to Shub-Niggurath's Pit.