No, Casualties: Unknown does not have official multiplayer: it is a single-player game, and the Steam store page lists Single-player as its only mode. The only way to play it with friends is Casualties Together, a free fan-made co-op mod that adds online lobbies on top of the full game.
Casualties: Unknown launched on Steam on November 9, 2026, from developer Orsoniks and publisher Oro Interactive. This guide covers what the official game includes, what the Casualties Together mod adds, how to install it, and the catches you should know about before you invite anyone into the caves.
What the Official Game Includes
The Steam store page tags the game as Single-player and Family Sharing, and nothing else. Orsoniks’ launch announcement and the 1.0 patch notes list what the full release added over the demo: layers 6 to 11, endings, two new playable characters, Endless mode, prosthetics, 40 achievements and a long list of fixes. Co-op and online play are not on that list, and the developer has not announced them for a future update.
That fits the design. Every run is one experiment, one body and one set of wounds to manage, and the whole game is tuned around you keeping that single character alive.
The Casualties Together Mod
Casualties Together is an online co-op mod hosted on Nexus Mods and GitHub. It is the most downloaded mod for the game by a wide margin, with well over 100,000 downloads on Nexus as of November 10, and it was built during the demo period before being moved over to the full game.
Its version 5.5.0, released on November 10, targets the full 1.0 release. The mod’s own release notes are honest about it: the update “isn’t thoroughly tested but it works from start to end”. Expect bugs and frequent updates for a while.
What the mod adds, according to its page and release notes:
| Feature | What it does |
|---|---|
| Steam lobbies | Host or join a run with friends through Steam |
| Voice chat | Real voice chat with in-game effects |
| Character select | Everyone picks a character and readies up before the run starts |
| Healing others | Right-click another player to interact with and treat them |
| Shared warmth | Standing close to each other shares body heat |
| CPR minigame | Revive a downed friend |
| Carrying | Pick up and carry incapacitated players |
| Spectator mode | Watch the others after you die |
| Pointing | Middle mouse button points at a spot |
| Separate endings | Different players can reach different endings in the same run |
It also changes a few rules for co-op: sleeping is disabled, hunger and thirst drain more slowly, and the unchipped option is set per player. Most of those can be tweaked through the in-game console.
How to Install Casualties Together
The mod runs on BepInEx 5, a free mod loader for Unity games. Every player in the lobby needs the same setup:
- Download the BepInEx 5 Windows x64 zip linked from the mod’s install guide (version 5.4.23.5 at the time of writing).
- In your Steam library, right-click Casualties: Unknown and choose Manage > Browse local files to open the game folder.
- Unzip the BepInEx zip straight into that folder, so its files sit next to the game’s .exe.
- Download the mod’s zip from its files page, open its
modfolder and move the contents into the same game folder. - Launch the game through Steam. New multiplayer buttons appear on the main menu.
On Linux or Steam Deck, add WINEDLLOVERRIDES="winhttp=n,b" %command% to the game’s launch options in Steam so Proton loads BepInEx.
To go back to normal single-player, press Deactivate MP Mod in Settings > General. Make sure everyone runs the same version of the mod, since a mismatch is the first thing to check when someone cannot join.
Things to Know Before You Play Co-op
- It is unofficial. Orsoniks does not make or support the mod. Report co-op bugs to the mod’s own Discord or GitHub, not to the game’s developer.
- The full-game port is new. The 1.0 version landed one day after launch, and its notes say the special crystal effects in one of the later layers sync unreliably for now.
- Back up your save first. Copy your save folder before installing anything. Our guide to how saving works shows where it lives.
- Sleeping works differently. Since the mod turns sleep off, your usual rest routine does not apply in co-op. Our sleep guide covers how it works solo.
- Reviving is on you. With friends around, a stopped heart is something another player can work on, and our guide to CPR in Casualties: Unknown covers the basics.
- Add-ons need matching versions. Mods built for co-op, such as skin changers and extra content, often list Casualties Together as a requirement, and every player needs the same ones.
Frequently Asked Questions
Is Casualties: Unknown co-op? Not officially. The game is single-player only, and co-op comes from the fan-made Casualties Together mod.
Does Casualties: Unknown have crossplay? No. It is a PC-only game on Steam with no official online mode, so there is nothing to cross over to.
Does the multiplayer mod work with the full game? Yes. Casualties Together 5.5.0, released November 10, targets version 1.0, though its notes say it has not been thoroughly tested yet.
Do all players need the mod? Yes. Everyone in the lobby needs BepInEx and the same version of Casualties Together installed.
Will Orsoniks add official multiplayer? There is no announcement of it. The launch announcement and 1.0 patch notes cover single-player content only.