Shang-Chi builds upon Iron Man 3’s Mandarin fake-out by revealing the real Mandarin to be Tony Leung’s Xu Wenwu, who is also Shang-Chi’s disturbed father.
Xu Wenwu is the leader of the Ten Rings, an army that toppled governments throughout history.
He is a very old character who has been in the fabric of the MCU for thousands of years, with Mandarin being one of his many names over the centuries.