This is a Doom 2 wad that takes place right after episode 3's ending and before episode 4 "Thy Flesh Consumed". 

This is a custom map made for Doom 2 so it will only work if you have the Doom 2 wad and to play it, I recommend using GZDoom since this map was made for that specific source port.  

If you are going to use GZDoom I recommend using the Voxel Doom mod with it because it makes the models 3d voxels which looks neat in gameplay. 

Here is the link to the wad page: Don't Kill a Man's Rabbit (Doom 2 Wad) by Fox Studios122 (itch.io)