The Ten Rings in the MCU is the terrorist organization that kidnapped Tony Stark back in Iron Man 1.
If you look at the flag behind Tony you might recognize the layout from the Shang-Chi announcement.
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Now, the Ten Rings were revealed to be the symbol of the (fake) Mandarin from Iron Man 3.
Now, in the “All Hail the King” one shot, we see the fake Mandarin (Trevor Slattery) kidnapped from jail by the real Ten Rings organization. The reason for this is that the real Mandarin (who was thought to be some underworld myth) is displeased at how Trevor impersonated him.
The Legend of the Ten Rings, refers to this actual underworld figure of the Mandarin. In the comics he is known for his ten alien rings, each ring giving him a different power. So the title indicates we’ll finally get the real Mandarin in the MCU.