I was looking for a fantasy roleplaying game without luck. Does anyone know such a system / setting?
As a reference portal fantasies serve when a person is transferred from one – usually our – world into another world. Narnia is probably the most famous example. There are also The Magicians books and TV series and Sliders, just to name a few.
I know I could do my own, but I'd rather use someone else's when the job is done.