"So that's what happened…"
Vital Weapons.
Weapons made out of the strongest desire of a creature. Their ambitions would manifest as a form of power, a weapon for them to manipulate to their desires.
The Spirit's greatest desire in life was to witness the most perfect form of a living being—the epitome of living creation. To see the utmost limit of a lifeform. Thus, her Vital Weapon was a Mirror that could help her possess any lifeform she could defeat in will.
Fitting for her dream, if one asked Han Li.
But that alone was not the only factor that affected a Vital Weapon. A creature's desires would evolve as they lived more and more. When you were 10, you'd want a specific toy, but when you were 20, you'd want something else.