Indeed
Really, Absolutely
adverb
If something is moving towards sincerity then it indeed and really did happen. Absolutely.
まことに
My 事 (まこと) really did happen! Indeed it did!
誠に申し訳ありませんが、当ホテルは全室禁煙となっております。
I am very sorry, but smoking is prohibited in all rooms of this hotel.