The Emerald Dragon - Dragomeir, #5

When an ancient warrior named Tanis is called by an emerald dragon - Demios, the first emerald dragon to be born in over a thousand years - he not only has to learn to be a dragon rider, but to interact with and eventually gain the trust of the other riders and people of Mt. Drago. The mountain lies in the middle of a desert, but has special access to other worlds; worlds inhabited by both unusual and evil creatures.
Tanis and the other riders, led by the queen of the dragons - The Ariella - must band together to create a fighting force and fly across the River Styx, beyond the Ice Caves to the Black Fortress. There they must retrieve the first of three crystal keys. These keys have held their enemies at bay since the first of dragon kind, but were captured by the Dark Lords of the Provinces. As The Ariella's riders fly over land and sea, they summon surprising courage to protect the dragons and Mt.
Drago from the vile forces which stand in the way of their peaceful existence.