The so-called Inner Strength is the process of continuously expanding one's Grotto Heaven until it becomes like a supergiant star, reaching the absolute limit of its capacity.
To expand the Grotto Heaven, one can rely on various external forces.
However, the most common method is to unfurl one's Domain to absorb the omnipresent Spiritual Material that exists throughout the cosmos.
Of course, the only drawback to this method is that it's too slow.
Although Spiritual Material is everywhere, capturing and containing it after unfurling a Domain takes an immense amount of time.
Ancient Divine-Demons like Sajia and Siming Tian broke through to become Tier Seven Life forms over five hundred thousand years ago, yet they remain stuck at this stage.
Therefore, many will resort to other methods, such as searching for Curiosities or Marvels that have the effect of expanding their Grotto Heaven.
