At this stage, one has officially stepped beyond the mortal world, and can travel into boundless space with no issues. At this stage, one will no longer use heaven and earth energy, but natural essence energy, far superior and more powerful. At this level, the immortal soul is true in strength and form, and one has officially gained one piece of immortality. A tribulation will accompany every breakthrough. This is considered to be the highest stage of cultivation in the Spirit Realm. The Three Human Sovereigns are at this level.
Cultivators enter the Body Integration stage when their Nascent Soul matures using World Origin Energy to the point where their original body "integrates" with their soul as an avatar. While cultivators at this stage have limitless lifespans, in practice, most cultivators at this stage don't live beyond several tens of thousands of years. This is due to the tribulations that occur every 3000 years. Each tribulation is stronger than the last, even if a cultivator does not progress in their cultivation. As a result, cultivators unable to continuously advance through the Body Integration stage are eventually killed by their own tribulations.
Powerful beings such as Devil Monarchs will attempt to disrupt a cultivators ascension to the Body Integration stage in order to prevent rival realms from increasing their strength