Defiance of the Fall

1. The Perennial Vastness scans each arrival, assigns a guide (Null for Zac), and starts him with Mana. Mana sets your Echelon and gives you domain-like influence over the realm, and the Cosmic Gallery runs for about 250 years outside (2,500 inside) with around 100,000 cultivators. His core-formation toolkit ends up being the Calamity Core, Fate Kernels and a Warstone.
2. He builds his reputation through duels and missions. The wiki lists a first shared mission with Catheya and Ogras at Sinner's Lament, a duel draw with Astara, run-ins with Holy Son Nerven Serku and Ventus Kalavan, and a Red Zone kill spree worth 1,000 Mana for five kills (Ch 1,010).
3. Princess Valsa Planur of the First Imperial Heaven, a Seventh Echelon guest, recognizes Zac as a Flamebearer and plots to seize his fate. Zac kills her in the Vastness (Ch 997).
4. Zac takes the Calamity Mountain global event, a Mountain Heart worth 5,000 Mana (Ch 1,004-1,005), and wins a Calamity Core almost perfectly aligned with his path. He is helped by Blood Clan scions Solomis and Ysaya Noz'Serasta and runs into the Earth-Dao team Sentinel Dawn. He mines Fate Kernels from the Quarry and later sells the last one to Astra Ar, a mage who also nearly beats him in a 'marital duel'.
5. He reaches the third reincarnation of the Nine Reincarnations Manual (Ch 1,007), putting his soul on par with elite Peak E-grade Mentalists.
6. Zac and Catheya spend leisure time in the realm. The wiki marks the Heavenly Cauldrons as their date location (Ch 1,019) and the Everfrost Isles as their future honeymoon location (Ch 1,024).
7. He meets the Realm Spirit Daoist Sendor (Ch 1,036-1,037), who explains the Vastness is a lab running experiments on how the Heavens heal, and that the realms are seeds of potential he gathered. A Warstone is chosen as the base of his core (Ch 1,037). Zac completes his first full cycle of Sealbearers (One by Nine, Ch 1,038).
8. The final stretch of the book, 300+ pages by reviewers' count, is Zac forming his cosmic core after a long series of challenges.

The book ends at the climax of his cosmic core formation, and reviewers describe it as an abrupt stop without a resolution. The wiki dates the completed Early D-grade core to Ch 1,060, which falls in the opening chapters of Book 13.

Peak E-grade at level 150, partway through forming his D-grade cosmic core. He reaches the 14th Echelon in the Vastness. The wiki dates the completed core (Early D-grade) to Ch 1,060.