The Celestial Realm is a supernatural realm.
After MK's team and Red Son agreed to team up, they went to the Celestial Realm to get the Celestial Artifacts so they can make an antidote to the Spider Queen's venom. However, they got into trouble when they ran into the Spider Demons and the Lion Guardians once they got the artifacts. Despite this, they were able to escape from them and leave the Celestial Realm.
In Monkie Kid