I watch anime in Japanese w/ subtitles (but in my language), and yoku (often) a question "hontou ni" translates like "True?", or "Really" (as I understand they're more sinomous in this case).
So, why "hontou NI", not just "hontou"/"hontou desu ka"?
I know that "ni" is particle uses for destination, answer like "where"/"whome" or uses with time (and days of the weeks).
But why here?

Maybe I heard wrong, and is it one word (hontouni), or is this another "ni"?

But on another hand, there's phrase "hontou arigatou" that means "Thank you very much" (or like "Thank you, really", and there's no "ni"...