Tsunehiko Watanabe 
The Ideal Sponger Life: Volume 15 (Light Novel) [EPUB ebook] 

Zenjirou and Prince Yngvi have been invited to Utgard, a city wrapped in a veil. Representative Rök—the city’s king, for all intents and purposes—welcomes them to the hidden city-state in the ice fields. He has an existential problem on his hands and sees Zenjirou (or rather, his lineal magic) as Utgard’s best hope.

Zenjirou, in turn, has a mountain of discussions and negotiations awaiting him. He must determine if and how the Capuan royal family will assist Utgard, and what they will receive in return.

Meanwhile, Prince Yngvi’s ambitions lead to his asking his father for permission to take a concubine from Capua, and since the king clearly sees the country’s value, it’s a very real possibility!