Reign of ash - Gail Martin

Blaine McFadden survived six years in the brutal Velant prison colony, exiled for murder.
When war devastates his homeland of Donderath, it also destroys the intentional magic on which Donderath and its fellow kingdoms rely.
Blaine and a small group of fellow exiles return to a lawless wasteland where unrestrained magic storms appear and disappear unpredictably, and monsters roam the ruins.

Yet rumours persist that the seeds of a new magic rest with a dangerous, ancient ritual and a handful of survivors who have disappeared. McFadden resolves to find these survivors and work the ritual, despite the danger, to restore the magic and end the chaos.
He rallies a small and desperate army for a last stand, knowing that if they fail, the civilisation of the Ascendant Kingdoms dies with them.