A good starting-point for writing good user stories is the book by Mike Cohn. There are also plenty of useful resources online how to write good user stories and how to work with the backlog as a Product Owner and how to involve the team to continuously keep the backlog in good shape so you can have effective sprint planning sessions, do reasonable forecasting of dates when features will be available etc.
It's always important to stay practical and experiment. Start with something and then inspect&adapt that so that you improve. This is the essence of any agile tool/framework.
If you want, start with the big epic and see how that goes.
Probably it's not worth having that epic, but rather discuss various things the player of he game can do that are more specific. These things are user stories, i.e. things that add user value throughout a whole product. A vertical slice of the system.
Some made-up examples to illustrate ideas around stories in a game setting:
- Player that gets 100k points will get rewarded with a golden medallion.
- Player must complete X and Y in order to open up the secret passage in Z.
- Player can use portal to teleport to any level he chooses.
Yes you can see it as a tree or more and more specific stories that are children of other stories or epics. Practically, you only care about the leaves as that is the product of working together as the team and PO to work on backlog items during sprint planning or other sessions.
Experiment and start with something and improve from there.