「・・・？」「、」
"Father, is this... a real blade?" "Of course it is, this is a duel."
Literal
"Father, this-thing [topic-って]... real-blade?" "Obvious [conjecture-だろう], duel [reason-なのだから-emphasis-な]"
Samurai drama dialogue. 親父 ('old man, dad' — rough masculine) is the son addressing his father. 真剣 has two meanings: literally 'real sword (as opposed to a practice sword),' and figuratively 'serious.' Here, given 決闘 ('duel'), it's the literal sword meaning. なのだからな is layered: な (predicative form of な-adj/noun) + のだ (explanatory) + から (because) + な (masculine assertive). Very rich sentence-final stacking.