It’s an Emperor Artifact with Space Principle. Inside the artifact is actually its own small sealed world. However, refining it and using this artifact requires the proficient in Dao of Space, and that’s why the world inside the artifact is so small cuz the sect master of the Emperor Star Mountain is not proficient in Dao of Space; he only knows a few things. And if a cultivator can refine and use this artifact, he can trap anyone inside, and inside the world, the user is the master of the world principle of the small sealed world, so he can control the world inside, he can kick anyone out. And the person being trap inside cannot escape no matter if he uses space crack like tearing space like Yang Kai cuz inside the world is its own world principle, so no matter what he does, he’s still inside… However, the usage is limited to how proficient the user is in of Dao of Space. So, the more proficient the user, the larger the world will become, and the more complexity in the world will become (there will be trees, mountains, rivers….) people can actually live inside. And the stronger the user, the more powerful the world principle will become.
Yang Kai will kill both of em tho cuz he has a powerful physical strength. However, it takes him some effort to refine the Sealed World Bead from the inside. Yang Kai will use it till he reaches Open Heaven realm. He uses it to grow his own medicines and herbs, uses it to store things, strengthens it by letting it devour nearly a whole star field (in Star Boundary arc, to strengthen its world principle) and accidentally having people from the stars being devoured live inside the small sealed world, uses Time Principle to create the flow of time inside(Star Boundary arc), devour most of the Demon realm (Star Boundary arc)… The final use is to strength his own universe world after breaking through Open Heaven realm by fusing it with his universe world (Outer Universe arc).
Yang Kai will kill both of em tho cuz he has a powerful physical strength. However, it takes him some effort to refine the Sealed World Bead from the inside. Yang Kai will use it till he reaches Open Heaven realm. He uses it to grow his own medicines and herbs, uses it to store things, strengthens it by letting it devour nearly a whole star field (in Star Boundary arc, to strengthen its world principle) and accidentally having people from the stars being devoured live inside the small sealed world, uses Time Principle to create the flow of time inside(Star Boundary arc), devour most of the Demon realm (Star Boundary arc)… The final use is to strength his own universe world after breaking through Open Heaven realm by fusing it with his universe world (Outer Universe arc).