Yes, it has been 20 years since Quake 1 was released in 1996 ! It was the one of first FPS (first person shooter) games that shook the gaming industry by the use of full real-time 3D rendering. After the Quake engine was made open-source by the id software (the makers of Quake), open-source developers made their own custom Quake engines like ezQuake, Quakespasm, DirectQ etc. The custom game packs soon followed. In fact, people never stopped playing Quake. They kept playing this addictive and legendary game.
To celebrate the 20th anniversary of Quake 1, MachineGames (developers of Wolfenstein – New Order) have announced on their Twitter account a new episode for the game. The new episode can be downloaded and installed inside the original Quake 1 folder and played from there easily. This works only with the full registered version of Quake as the shareware version of Quake does not allow custom game mods.
Suppose you have Quake 1 installed inside c:\quake folder. Then all you have to do is download the RAR file https://cdn.bethsoft.com/quake/dopa.rar and extract the contents to the Quake folder. Now you will have a folder dopa inside c:\quake -open this new dopa folder and double-click on the dopa.cmd file. This will launch the new episode using WinQuake.exe.
Even though the original WinQuake works alright and you can play the new episode fine. But the graphics quality is not very good (on Windows 8.1). So you may want to try something like ezQuake or Quakespasm. In these cases, you will have to edit the dopa.cmd file inside the dopa folder and change the line winquake -game dopa to ezquake-gl.exe -game dopa or quakespasm.exe -game dopa etc.
As far as the new episode is concerned, it looks like some secret facility of World War 2 times. The vicious attack dogs and special skull symbols indicate that perhaps you are entering a NAZI military base. You have to kill the vicious attack dogs, save yourself, find secrets and open the doors using hidden keys and switch sequences. Great fun for the Quake lovers!