Death and Dormancy:

 

It is not impossible to kill a Celestial but it is very difficult to do. They're also not technically being killed, they're simply being wiped from existence. To kill a Celestial there are two methods. One requires a body that once housed a Celestial soul but no longer does.

 

First, a Mortalborn Celestial must be in a dormant state with an inactive soul. This Celestial must then have the soul removed by killing the body then reanimating them with magic which preserves them in the last state they were in. This is often done by removing the heart, the source of magic, then replacing it with a preservation magic. This is generally not possible with non magical beings as there needs to be a way to reanimate the host body.

 

Once this is done, a different Celestial can occupy the body similar to a possession. While they’re in the body, if the heart is replaced and the Celestial restored, the host soul will absorb the occupying Celestial soul. Destroying their body and all memories preserved in their soul Essentially wiping them from existence. This will also cause the body to become a new Celestial with the body plan of the one that they absorbed. 

 

The second method is for a cosmic Celestial to absorb or reabsorb the soul of another which functions much in the same way but can only be done by the cosmic type.

 

Celestials can also enter a dormant state. If the soul is damaged, broken or removed, the body enters a dormant state. They can no longer access the immense magic they once could and will often forget who they are entirely unless they are born dormant. In which case they will simply build fresh memories. They can be awakened by repairing the damage done to the soul and then accepting that they are in fact a Celestial. This can happen to both Mortalborn and Starborn. However should this happen to a Mortalborn and they die before they can reawaken/awaken then their soul will search for a new body developing nearby that it can take over. Should this happen to a Starborn their soul will shatter and they will need to reform.

 

The last form of destruction a Celestial can undergo is shattering. This is the easiest way to defeat a Celestial but is not permanent. When their soul is shattered, their body will disappear into dust and their soul will break apart. It then travels to the nearest cosmic Celestial where it’ll remain until it is reformed and ready for a new body. While this is the easiest and least convoluted method, it is still extremely difficult for mortal beings to accomplish as it requires a substantial amount of magic. Or just raw physical power that prevents them from healing the wounds given to them.