Diablo 4 lead clarifies how campaign co-op progress will work

gamesradar.com:

Diablo 4 looks a lot of fun when playing solo, but even more so when you add friends to the mix, and with the action-RPG now less than a month away, general manager Rod Fergusson has taken to Twitter to clarify exactly how campaign progress works when you've got some buddies by your side.

"The host of the party owns the state of the world," Fergusson explains. "If you're aligned (aka at the same point in the quest line), then you all progress. If there is a misalignment (like you invited someone new halfway through), then they won't get the story quest progress/xp until they catch up."

So if you're playing with a group that's not committed to playing together, it's always best to have the player with the least amount of progress to be the host.May 15, 2023

With this in mind, the Diablo 4 developer advises that if you're with a party that doesn't consistently play together, the player with the least amount of progress should host the game so they can benefit from the quest progression and experience.

Of course, while having a group that's similar in strength is ideal, it's not essential, as Blizzard has gone to great effort to ensure that the game is enjoyable for all, even if you happen to be a much lower (or higher) level than your teammates. 

"One of my favorite features is the fact that the game scales individually for players," Fergusson says (opens in new tab). "So questline progress aside, you can have a level 1 and a level 30 playing together and having fun because the game scales for them and the new player doesn't have to hide in the corner."

The latest Diablo 4 beta, or "server slam", was held last weekend, giving fans another chance to try out the game ahead of its launch on June 6. Players were able to test