What they did with Arum, is Arum is the world. people claimed a section of the land in the world to build their game story in. there are some hard-set rules, like, cant kill another authors main characters in your story type things, but you can reference events in another authors stories, and how it may have effected your games story.
Author A has a kingdom with a prominent trade port, but for a while, the port gets sieged and ruined.
Author B has a small land not to far from Author A, and usually has to import most of its needs. When the port is taken out of commission, famine and hardship spread across this author's land.
Another instance, in my game I'm working on, there is a large organization across all the lands referenced as 'Guild.' While not normally considered a military organization, they work within various kingdoms borders, taking on tasks, for money, that local militia can't handle, don't want to handle, or the military just doesn't care to handle. A group like that could appear in multiple lands, or even a common enemy, like "The Hoards" that live in lands no longer considered Civil, and they raid various kingdoms, pillaging items and people, who are lucky if they are enslaved.