In a tweet from the official Twitter account for the STALKER game series, developer GSC Game World officially announced that it is using Unreal Engine 4 for the upcoming sequel STALKER 2. The previous games, which are several years old at this point, used the proprietary X-Ray Engine that allowed those games to stand out visually at the time with stunning lighting, shadows, and weather, but it seems that Unreal will make things simpler for the devs.