Pure chaos, that's what Mycopunk is. It's exceptionally bizarre and now my new favourite first-person shooter because it's so wildly unique. Note: key provided to GamingOnLinux.
I was already quite impressed with the demo and now after playing more, I'm hooked. It's very much a wonderful combination of elements from the likes of Risk of Rain 2 for the ongoing chaos during missions and Deep Rock Galactic for the structure of it only with the weirdness turned up to maximum level.
You pick a character class, and then go solo or team up with others across various different types of missions in various biomes across a planet. Missions can range from simply crushing as many enemies as possible, to building pipes across the map to connect points up and all sorts in between.
The different classes you get are quite fun but it's the enemy variation that really makes this completely nuts. Enemies are constructed of various parts you blow up and they fall apart, but other enemies might pick up their parts and attach it to themselves.

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Half the time I don't entirely know exactly what's going on, because it really does just turn into pure explosive chaos. You have enemies appearing from everywhere, and some of them are massive freakish creations that fire all sorts of weapons at you. Some can be destroyed from afar, some need you to get up-close and personal. So it's good you have your own varied arsenal of weapons too.
There's lots of different weapons but you also get a customisation system. As you level up your character and weapons individually, you get access to various upgrade modules. You can place lots of them on your weapons in a no limits grid-based system, so as long as they fit together you can use them. It makes creating a build very unique with each player potentially being very different even when playing as the same class.
Quite funny that downed team mates need you to piece them back together, as you run from their legs to their torso to bring them back into the game. This will happen a lot if you're on my team, because friendly fire is most definitely a thing here. And with all the action going on, it's a bit too easy to annihilate your friends (sorry!).
Fantastic atmosphere too. The visual style in especially odd but in a good way it all fits together, and it definitely leans into the comedy side of things with some of the interactions and voice-over.
Mycopunk works perfectly on Desktop Linux with Proton 9.0-4. Valve also rated it Steam Deck Playable, the only issue being noted is you need to manually bring up the on-screen keyboard.
It's in Early Access so everything is subject to change and more content is to come, but what's there right now is already a huge amount of fun and an easy recommendation from me. A great choice to run around in with a couple of friends.