There are several first-party PlayStation studios making games in Horizon Forbidden West's game engine.
Decima is Guerrilla Games' in-house engine, first used to make the PS4 launch title Killzone: Shadow Fall, and later used for games including Horizon Zero Dawn, Until Dawn, and Death Stranding. An updated version of the engine was used to develop Horizon Forbidden West, and it's now being used by multiple PlayStation studios for upcoming games.
This was revealed in a recent Famitsu interview (machine-translated) with PlayStation Studios head Hermen Hulst, who was in the middle of answering a question about collaboration between studios when he said this:
"Of course, there is also the sharing of technology. For example, there are several studios using Guerrilla Games' Decima engine. We also place importance on improving our technology, and we share that idea among the studios."
Hulst didn't reveal exactly which games are in development using Decima, but whatever Kojima's cooking up next could be one of them. There's the rumored new horror game Overdose and then the much murkier, but still very much rumored Death Stranding sequel. Until Dawn studio Supermassive's most recent effort, The Quarry, was developed in Unreal Engine 4, but it's not outside the realm of possibility that it's working on something new in Decima.
We do know PlayStation is making a big push into the PC, mobile, and live-service markets. Hulst said in a recent interview that PlayStation Studios' live-service games could launch simultaneously on PC and PlayStation consoles. That said, he's assured multiple times that PlayStation isn't straying from the single-player narrative games it's best-known for producing.
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