One of the unique modes available in Doom Eternal is Invasion. By playing in this mode, you can become a demon and enter (jump into) the campaign of another player. The following page of our guide contains the most relevant information associated with this gameplay mode.
Invasion enables you to make the single player mode much more difficult. It allows you to become a demon and make it more difficult for other players by interfering in specific mission of their story campaign. In other words, you can intrude into their gameplay as a demon. When you're playing the game, you may see a pop-up message that other player has just appeared your game.
A demon controlled by another player is marked in a unique way. Please note and remember that this character is not controlled by AI. That's why it this particular monster won't act like a typical opponent. This has a significant impact on the game's difficulty level.