The Magician Who Rose From Failure: Volume 4 by Hitsuji Gamei

Arcus came to Rustinell to top up on supplies; that’s still on his to-do list, but he has averted a plot to kill the crown prince of Lainur! The prince declares war on the treasonous Porque Nadar—and orders Arcus to fight by his side. Arcus never meant to get involved, but a monarch’s word is law. After all he’s been through, Arcus might be prepared for open warfare; regardless, there’s no place more dangerous to stand than in the shadow of the boy who would be king. Especially when you consider the royal family’s track record with close friends...