> What is this?
It's cave story, deathmatch. Someone made it possible to play it in COOP multiplayer, and I made a basic mod for deathmatch. Somewhat similar to old arena shooters, so it's a little more dynamic and similar to how you get upgrades and weapon levels in singleplayer.

> I haven't even played Cave Story.
https://www.cavestory.org/

> Cave Story has multiplayer?
https://cavestorymultiplayer.com/index.php

> Guide
https://pastebin.com/TvjpcTQT

> Download
https://www.mediafire.com/file/dcofs294b7gctbn/CSDM_Files.7z/file

Refer to the Guide on installing it.