Senki to Yobareta Otoko, Ouke ni Ansatsu Saretara Musume wo Hiroi, Issho ni Slow Life wo Hajimeru summary:

The kingdom's mercenary Ordoll, also known as the "War Demon", is feared by enemies and allies alike.
Although he is meant to win every battle, he is "assassinated" the day after a cease-fire agreement with a neighboring country is concluded.
After pretending to be dead, he manages to escape and decides to head home, shaving his beard and shortening his long hair.
Immediately before arriving at the village, Ordoll picks up a five-year-old little girl who calls him her "papa".
The strongest mercenary, who suddenly becomes the father of a 5-year-old child (♀) even though he isn't married, eventually develops into the strongest parent!