Following the GameSpy server shutdown in 2014. Damnation can no longer be played online natively. OpenSpy[openspy.net] is an open source GameSpy emulator that as of July 2021 fully supports the multiplayer functionality of this game, and many others.

If you want to unlock your frame rate too, see my other guide - https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=2551388580

Firstly navigate to your game directory by selecting Damnation in your library, right clicking on it and hitting properties, then selecting browse under the local files tab. Click on the binaries folder, and you should see "DamnGame.exe". Back this file up by renaming it to something else, e.g. DamnGame.exe.bak.

Next, download and extract the patched Damnation executable[drive.google.com] and copy it over to the same folder. Your folder should now look like this.

From the main menu, select versus if you want to play versus multiplayer or campaign if you want to play co-op.
Now select online.
Press F1 and create an account.
And then you just sign in and host and join matches as normal.