https://www.pcgamer.com/doom-eternal-invasion/

Id Software dropped a big surprise about Doom Eternal today at QuakeCon when it revealed that players can "invade" games, taking control of demons to bring down the Doom Slayer. They can go one-on-one, or team up to form a "slayer hunting party."

We'll cover more details about what's coming, but this is a big, big change, and it reflects a new approach to the game: "We're not just making a Doom game anymore," id's Marty Stratton said. "We're building a Doom universe."