The Emerald Dragon. Dragomeir, #5

When the ancient warrior Tanis is summoned by Demios, the first emerald dragon born in over a thousand years, destiny unfolds. Drawn to Mt. Drago, a desert citadel riddled with gateways to other realms, Tanis must prove he is more than a forgotten swordsman. To stand among dragon riders, he must earn the trust of hardened warriors and legendary beasts while navigating rivalries, oaths, and dangers that reach far beyond the desert mountain.
Under Ariella, Queen of the Dragons, the riders come together as storm clouds gather because the Dark Lord of the Provinces is rising, and his shadow stretches across every world linked to the mountain's gates. The riders' only hope is an ancient set of three lost crystal keys, forged to seal evil behind unbreakable doors. The first lies beyond the River Styx, past the Ice Caves, and within the iron heart of the Black Fortress. Now Tanis and the riders must fly farther than any legend dares to name over savage seas and cursed lands, facing monstrous guardians and trials that break the unworthy.
For if they fail, it won't be one mountain that falls. It will be every realm that breathes on the other side of Mt. Drago's gates. Legends awaken. Dragons rise. And one warrior must become the storm.