This mod includes a new room for Enter the Gungeon, (you can only load it if you have the custom rooms mod by Kyle in https://modworkshop.net/mod/27087#files) made with the room creator.
Go to the link in the description and download only the second file (and if you want to make your own rooms the first one also).
Copy the zip archive (don't unzip it) and paste it in your computers directory: 32-bit C:\Program Files\Steam\steamapps\common\Enter the Gungeon\Mods or if you're in 64-bit look at C:\Program Files (x86)\Steam\steamapps\common\Enter the Gungeon\Mods
Then create a folder called CustomRoomData at 32-bit C:\Program Files\Steam\steamapps\common\Enter the Gungeon or 64-bit C:\Program Files (x86)\Steam\steamapps\common\Enter the Gungeon
If you're on epic games do the same in the epic games directory
then download this file, copy it and paste it at 32-bit C:\Program Files\Steam\steamapps\common\Enter the Gungeon\CustomRoomData
and 64-bit C:\Program Files (x86)\Steam\steamapps\common\Enter the Gungeon\CustomRoomData
Once you download it you can play. Be very carefull with that room, it's so difficult!