The Primordial Immortal Body is one of the Top 5 (Primal Celestial Bodies) from 99 ranked Sovereign’s Celestial Bodies.
When Mu Chen obtained ownership of the Primordial Immortal Body, he was not able to utilize its full power until he fused with the body.
To fuse with the Primordial Immortal Body, Mu Chen has to shed off his mortal physique and refine an Immortal Physique. To refine the Immortal Physique, Mu Chen must use the vast amount of Immortal Origin contained in the Primordial Immortal Pagoda and go through a body reconstruction. With the Immortal Physique, Mu Chen would have the capability to contend against Saint Heavenly Sovereign just with his physique alone.[1] The Immortal Physique wasn't genuinely indestructible, it is just a higher level of physique refinement.[2]
One of the Sovereign Abilities of the Primordial Immortal Body is called the Primordial Immortal True Seal Sphere, which is comparable to the 36 Peerless Divine Abilities. This ability harnesses the power of Immortal Origin to form it into a sphere and anyone that got trapped inside the sphere was subjected to a rushing flow of time, in which they would have a hard time breaking through and would eventually be turned to ashes by the passage of time.[3]