«The End of Magic» by Mark Stay

‘What will you do when the magic is gone?’
Sander Bree is a royal mage. Steeped in privilege, he lives a cushy life advising the king on matters of court and politics, yet still complains that he's stuck in a rut.
Rosheen Katell is a freelance mage and, with Anzu her griffin, she’s worked hard to build a reputation as a trustworthy truth seer. She never lies, never kills. 
The source of their power is the Lapis Moon in orbit above. Very soon, that magic will be gone, changing their lives and their world forever. Sander must keep a promise that would have been difficult enough with magic, but is a suicide mission without it. Rosheen is forced to side with a murderous warlord, and her once-solid principles are tested and found wanting. Both will be set against one another in a war unlike anything the world has seen before, in this sweeping fantasy of magic’s ending.
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‘Mark Stay paints his world and characters with deft, confident and dark strokes.’ — RJ Barker, author of the Wounded Kingdom Trilogy
‘The sort of book you give to people who say “prove to me why I should read fantasy”… A simply terrific read.’ — James Barclay, author of the Legends of the Raven series
‘Takes the tropes of high fantasy and uses them to wield an original, compelling and intricately plotted story… exactly the kind of fantasy that we need more of.’ — Gavin G. Smith, author of the Veteran series