Riftwalkers

There is place called the Rift. It is a bottomless canyon with catacombs all along the sides. Each cave leads to a different reality and a different world and a different time. In one of them, you and I live. In another, knights still battle dragons. Not all is at peace between the realms. An organization, called the Riftwalkers, collect heroes from each of these universes. They work to keep the balance between the realms and to make sure that everyone stays in their own world, especially those wishing to do others harm. In the little islands that float in the void, a colony of Riftwalkers, in training and off duty has gathered. It is up to them to protect the Rift from unwanted visitors.

Disclaimer: I do not own Mistborn (by Brandon Sanderson), Harry Potter (by JK Rowling), Percy Jackson (by Rick Riordan), Rangers Apprentice (by John Flanagan) and many more books tba.