After rearranging the information about the Heaven Stealer Cultivation Technique, Ace finally understood what the system was talking about.
The Architect of Thieving Heart was all about cultivating the Qi and Soul while strengthening the Cultivation Heart. The Cultivation Heart also contained the inner space, which is now called the inner world.
He needs to absorb the Heavenly Spirit Qi or Primordial Spirit Qi into his soul and comprehend the prime law. Thereby, he expands the inner world with the prime law and assimilates other auxiliary laws until it becomes a real world.
Furthermore, his inner world's prime law was the Darkness, which was also the attribute of his inner world, and calling the Darkness World wouldn't be wrong. The First Realm of the Ascension Realm was called the Law Soul Assimilation.