The Game itself will be Single Player first and foremost. However, once the main game will be complete and I'll have the resources to do so - I would like to include separate PvP arenas and instanced Dungeons you can do together. These areas will be separated from the main world by a Loading Screen, meaning that they are not part of the main world.
However, you would be able to get useful rewards and items that would help your Single Player journey.