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

The lie must die was the warning Brekszta gave Alex, and she thinks she might now understand what the Old One meant. Traveling through the Sídhe branch of the Tree of Reality, the mages unravel the history that led to the first invasion of Earth. The threat of the Darkness looms with only theories of how to stop the disastrous force before it destroys all worlds. A decision awaits the mages at the dead homeworld of the Sídhe where it all started.

Merlin and Morgana have survived for millennia by adapting to each new era and area they lived in. In times of crisis, they fought back the enemies of Earth with their magic. But such a life takes its toll even on the strongest souls.