Ben Kingsley played the Mandarin in 'Iron Man 3', but it was later found out in the film that he wasn't actually the villain of the film
We hear it from Marvel Studios head himself, Kevin Feige. There are plans to bring back the real Mandarin and the Ten Rings in future Marvel films. Feige mentioned this during his recent Ask Me Anything on Reddit. He confirmed that they want to follow that plot thread that was unspooled in Iron Man 3. Your guess is as good as ours where they plan to introduce the characters might pop up. But as ComicBook.com points out, one possible place they could show up is in the Disney+ series, Falcon and the Winter Soldier.