Categories of Boss Powers
There are 24 Boss Powers in total, divided into Diablo 4 Items three categories:

5 Legendary Boss Powers

9 Rare Boss Powers

10 Magic Boss Powers

These powers can be obtained from bosses in Lair Bosses (available in Torment 1 and above), World Bosses, and the Apparition Incursion event. You can earn Boss Powers across all difficulties, but Lair Bosses are limited to higher-tier difficulties.

Main Powers vs. Modifier Powers
Main Powers: These are the core abilities that grant major effects. Main Powers can drastically change how you play, such as transforming your Evade into an elemental explosion or turning your channeling attacks into an ice beam. These powers are especially valuable for specific builds, such as Sorcerer Incinerate builds, which benefit from large-scale elemental attacks.

Modifier Powers: These provide passive bonuses or additional effects that support the main ability. They can boost your stats, enhance your existing abilities, or trigger secondary effects. Modifier Powers are the supportive aspect of Boss Powers, and they can synergize well with Main Powers for even greater effectiveness.

How to Use Boss Powers
To set a Main Boss Power:

Place the power in the top-most eye in the middle of the Boss Power menu.

The three slots beneath the Main Power slot are for buy Diablo IV Items Modifiers. You can place your Modifier Boss Powers here to complement your Main Power.