J.M. Briggs 
The Iron Path [EPUB ebook] 

One of their number is gone, and the threat of the Darkness looms over the mages as it slowly consumes the Tree of Reality. With so much at stake, there is only one path forward, and it will take Alex and the others to worlds where no human has ever walked. With the aid of the Dragon Emrys, the mages learn firsthand the dangers that have sent so many species fleeing to Earth as they fight to stay alive far from home.

For Merlin and Morgana, their immortality represents responsibility and grief. As the world moves on, they continue to fulfill their magical roles, threat after threat. But some stories are repeating themselves.