4A Games have done something quite awesome. They've released a SDK for people to make full game modifications of Metro Exodus.
This isn't some basic level modding abilities either, they've gone the full mile and released their own editor that enables people to create fully standalone content too. It has integrated mod.io support as well, so it's not tied to Steam. They said it includes "some tutorial levels and as an example of a fully featured, shipped level, we’ve included a full unpacked Metro Exodus sandbox level for you to tear apart using our tools".
One thing to be clear on though is that they're not entering the game engine licensing business. So, you cannot sell your works with it or put it behind any kind of paywall. This is just something they decided to do for their modding community. Pretty fun to see another game engine going a lot more open like this. Hopefully in future they'll perhaps open it up even more if this goes well.
The Exodus SDK only has a native build for Windows but we know their editor is clearly able to make Native Linux builds too.
You can buy it on Humble Store or Steam - both stores have it with 75% off right now too.
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