Doom Eternal Invasion is a single player mode feature that lets you invade (or be invaded by) other players. Here's everything you need to know about the Invasion mechanic.
Doom Eternal Invasion: Everything You Need to Know
Much like Dark Souls, players will be able to invade other player's campaign modes if Invasion is enabled. In an interview with Qconforums, id Software Creative Director Hugo Martin said Invasion will "add a dynamic element of difficulty for those who are looking for it."
The feature is meant for the late game, adding a twist to a player's second run.
The demons you can play as will have their PvP abilities with the exception of being unable to summon demons. The demon choices will vary by level. Martin suggests that you should be well versed in the level before invading other players or allowing yourself to be invaded. You could just play with Invasion mode on from the beginning as well.
Doom Eternal releases March 20 for PS4, Xbox, PC and Stadia. It will see a Switch release sometime in 2020.