The crucial damage done to Sukuna might be through his Gojo's Hollow Purple ... could have been something even greater. Yuji's domain expansion was hyped among fans for a very long time.

Yuji awakens a domain expansion to battle Sukuna ... Sukuna came out second best and suffered great injuries from the misfired Hollow Purple. He was then targeted by Hana Kurusu, and although ...

However, it is this self-awareness that has propelled Yuji into becoming a stronger character, even allowing him to unlock the mysterious Domain Expansion, the pinnacle of a Jujutsu Sorcerer.