Sareki Oukoku

Sareki Oukoku summary:

A kingdom in the middle of the desert is the last city of humans that were against the Sand People. However, their next king, Kirameki, didn't want to fulfil the throne. The Sand People took the chance and kidnapped the Prince and replaced him with an imposter, Kirameki's half-brother, Saga, who is a half-human, half Sand Person. Kirameki was made to do hard labour then, but he came back on the day of the Water Festival to get back his Kingdom with Serafita (the Water Goddess)'s help.