Taida no Ouji wa Sokoku wo Suteru: Koori no Majin no Tousouki

The small Land Kingdom found itself at war with the Froisel Empire, one of the continent's greatest powers. With no way to resist, the decimated Land Kingdom army entrusted the rear guard to the fourth prince, Lut, an illegitimate son, to allow the main force to retreat to the royal capital. Prince Lut, known as "The Incompetent," led a mere fifty-three soldiers. While victory seemed impossible, Lut harbored a secret: he was a Divine Rank magician, said to be one of only a handful in the world! Though Lut overwhelmed the enemy commander by demonstrating his godlike power, he and his fellow soldiers wished to surrender to the empire. This is the tale of the strongest ice wielder who abandons his homeland to become the empire's guardian deity.

(Source: Futabasha, translated)