Bonus Critical Strike Damage:

For each Vigor point spent, you’ll receive an additional 1–3% Critical Strike Damage. This bonus multiplies the effectiveness of Diablo 4 Items in your Critical Strikes, giving you a significant damage boost as you deplete your Vigor.
Summary of the Unique Effect at Max Vigor:
When Core Skills are used at max Vigor:

Damage is fully restored to its original value.
The area of Effect is increased, hitting more enemies.
Critical Strikes are guaranteed with every use of Core Skills.
Additional Critical Strike Damage is granted, based on the amount of Vigor spent.
This unique effect makes the Rod of Kepeleke an incredibly potent weapon, especially when you’re able to buy Diablo IV Items and manage your Vigor efficiently. It turns your Spiritborn’s Core Skills into devastating, high-damage abilities with guaranteed critical hits, making it one of the most valuable items for this class.