Sennen Sensou Aigis: Eiyuu no Kizuna

Long ago, the God Aigis sealed away the devils that caused havoc in the human realm. Now many thousands of years later the Demon God, Tiamat, has come to the human world seeking to destroy all humans using the very devils that Aigis had sealed away so long ago. One of the first to fall was the imperial capital where their prince, who is descended from heroes, had no choice but to run away and watch as his homeland burned. Now he must do what he can to survive Tiamat's onslaught and recruit allies in order to take back his kingdom. Will he live to see the day his kingdom is free of Tiamat's hands?!

Source: My Anime List