Shinwa Densetsu no Eiyuu no Isekaitan

Shinwa Densetsu no Eiyuu no Isekaitan (神話伝説の英雄の異世界譚) is a light novel series by Tatematsuri with illustrations by Ruria Miyuki.

Plot
Ouguro Hiro was once summoned to another world called Aletia, and while known as the God of War (Mars) he and his comrades saved the country and conquered the surrounding countries to form a large empire.

Following that, he made the decision to leave everything behind, and with his memories as compensation, he returned to the the present. Hiro was blessed with happy days, but due to some fate he was summoned back to the other world.

—there was, an Aletia after 1000 years had passed.

In a world where its past glory become "myths," a young boy known as the Dual Black Hero King opens the curtains to a new Mythical Legend!