Chaos Knight, book 2: Lance of Earth and Sky
Erin Hoffman2012

In the sequel to Sword of Fire and Sea, Vidarian Rulorat faces the consequences of opening the gate between worlds. Called into service by the desperate young emperor of Alorea, Vidarian must lead sky ships in a war against the neighboring southern empire, train the demoralized imperial Sky Knights to ride beasts that now shapeshift, master his own amplified elemental magic, and win back Ariadel - all without losing his mind.