Where does Owari no Seraph leave off in the manga?
According to Japanese Wikipedia, Wikia and many Reddit threads (e.g. this), the anime ends at chapter 41 (volume 11).

For the sake of completeness: season 1 ends at chapter 25 and season 2 ends at chapter 41.

However, as it has been mentioned on Reddit and explained on Wikia, the anime adaptation has some differences compared to the original work (manga).