Here are some things to think about in order to get a smooth co-op experience:
Steam!
The game now supports the usage of your Steam friends list which means you can skip the steps on how to do Port Forwarding and how to use Hamachi! However, I'll keep them here in case you run into problems and want to try the old fashioned way. SO, if your game tells you to enter an IP Adress you need to go back into the main menu and change the network settings from IP to STEAM in the Options menu. After this is done the game should now go straight into the lobby and in there you can go to Invite to bring up your friends list inside the game. You could also open up the Steam In-game overlay to invite your friends or join a friend that has a lobby set up already.
Port Forwarding & Hamachi
If behind a router, the person hosting your game needs to have
Port 6000 open, otherwise it will be
impossible to connect. For a guide on how to open ports, check
this thread.
As an option to port forwarding, there's
Hamachi. Hamachi is a free software that allows you to bypass port forwarding if everyone connects to the same group in the program. We set up a guide for using Hamachi in
this thread as well.
Avoiding Lag
If your internet is slow or you're downloading something, it might cause lag. It's probably wise to let the person with the best internet host your game if possible. Also, the further away you live from one another, the more base ping you'll get. This means playing with someone on the other side of the world (or even a few countries away) might be kind of laggy, depending on your connection.
Finding people to play with
If you couldn't convince your friends to join in on the fun, or you wanna make some new ones, you could always make a post on this forum asking if anyone's interested in playing! I recommend that you play with people on the same continent if possible. Let people know in the title of your thread what country or continent you're from or what language you're using to make things easier!
What happens to my Co-Op character when I return to Single Player?
If you were the host, nothing happens at all. Your quest stats and all your items are saved and you can just continue the game as usual.
If you were a client, your items and character progress will be saved, but you will (currently) have to redo any quests you finished in the co-op session. In the future we'll add a system which makes it possible to skip quests you finished in co-op!
Enemy Scaling
Aside from getting stronger in general, some enemies also get new attacks when in co-op. Many of these attacks are crowd-control based, meaning they might root someone to disable them from attacking! Sometimes you may be able to help your friend by hitting the enemy or their prison. Try it out!
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If you have anything to add or ask, go ahead and reply to this thread!