Sure, there is an Overlord manga. It expands on the story and characters from the original light novel series, giving fans another way to enjoy the world of Overlord.
No, they're not the same. Manga is a broad term for Japanese comics, covering various genres and themes. Hentai, on the other hand, specifically refers to pornographic or sexually explicit manga.
It depends on the laws and regulations of the specific country or region. In some places, hentai content may be restricted or outright forbidden, especially if it involves extreme or illegal themes.