isekai novel, guy is op meets a girl who feeds him alcohol and "does the deed" with him while he is out of it. she is a princess of a different country. later the country gives him the title hero but some anti-hero faction sends assassins after them. the flee but get ambused because a dagger the local nobel gave him appearantly has a tracking magic mark.
29-sai Dokushin wa Isekai de Jiyuu ni Ikita……katta maybe? I read a bit of the manga and the beginning is similar, tho I don't remember anything about a dagger