You can download Unity, purchase and install for example UFPS that will handle most FPS functionality for you (includes basic gun etc model) and you can use Unity networking for LAN connectivity.
Let's say the twist is a Battle Royale game, you just make a Unity terrain and a basic sphere that becomes smaller.
After that coding the extra bits such as HP, HP loss due to sphere etc isn't a big deal.
That's one way, doing it via a mod also works, but you really need knowledge of the product you're modding (although the same is true of e.g. Unity, if its your first week with it it won't be that easy).
You can download Unity, purchase and install for example UFPS that will handle most FPS functionality for you (includes basic gun etc model) and you can use Unity networking for LAN connectivity.
Let's say the twist is a Battle Royale game, you just make a Unity terrain and a basic sphere that becomes smaller.
After that coding the extra bits such as HP, HP loss due to sphere etc isn't a big deal.
That's one way, doing it via a mod also works, but you really need knowledge of the product you're modding (although the same is true of e.g. Unity, if its your first week with it it won't be that easy).