Scepter of Mercy Series by Dan Chernenko, Harry Turtledove

Since the Scepter of Mercy was lost from Avornis, the kingdom has been at risk from its magic-using neighbors. King Lanius and King Grus retrieved the Scepter in a tale of great triumph and adventure. But the true story of their dual reign has never been told. Until now...

The longer the Kings go without acting on their dream of retaking the Scepter of Mercy, the greater the advantage the Banished One gains. King Grus will have to act quickly-before his people realize just how formidable an enemy the Banished One truly is...

For hundreds of years, the rulers of Avornis sought to reclaim the Scepter of Mercy, their only protection from the fallen god known as the Banished One. They all failed. Now, the burden rests on the shoulders of two kings, divided by pride, but united in duty.