[Kyureki]

Spoiler for WN vol 2 spoilers/light summary:

The MC's party gets forced into taking a job by a guild master to be scapegoats, and as a result, get involved in a civil war with the only way to prove their innocence being to take part in the war and have princess support their opinion in the matter. Long story short, MC becomes known and feared for being a ruthless and demonic strategist, bringing the faction he supported from the verge of annihilation to winning the whole war. In the background, the whole civil war was a plot by the country that had summoned the MC (with the guy who planned it being that mage he killed in the beginning) to weaken the countries in that region so that they could easily conquer them afterward. The MC unknowing almost ruining that plan for them has their secret agent suggest to the princess the MC helped into granting him peerage and territory (the territory being one that's part of the country in name only as no citizens live there, only the indigenous tribes, and swarming with powerful monsters, not to mention it's almost entirely bare wilderness) to bind him in place, which the princess jumps on because she was frightened of the MC due to the abilities he had displayed during the civil war and was sure that she would definitely lose if he were to turn against her. The MC's response to this is that since she went back on her initial promise to let him go after everything is over, he plays things up so that his territory is practically completely independent from the princess' country.

Side note, this is where the reason behind the name of this series comes from. The territory he gains is the Wortenia Penninsula.