As mentioned in the past, the Mortal Rejection Realm was when one transformed the remaining part of their body into a Godly variant, the Soul!
Godly Energy quickly fused with Krune's Soul as it began to slowly change its own properties. Unlike what others thought, it wasn't that painful. Not to mention that Krune was a wisp, which was also considered a Soul Race. Of course, that made things far easier for him.
Time passed as his Soul continued to change. If not because of the Godly Energy entering his body nonstop, it would be quite a silent breakthrough. That just showed how smooth it worked. Eventually, the last strand of Krune's Soul changed as his cultivation began to rise. His foundation space increased in size, making his Universe Foundation even bigger.
It was then that a burst of Godly Energy came out of Krune's body, spreading everywhere around him. Krune had finally reached the Mortal Rejection Realm.