Diablo IV's Codex of Power Aspects are passive buffs players get Diablo 4 items  from completing dungeons. To imprint them on gear, players must visit Occultists.

Diablo IV’s Codex of Power Aspects are powerful upgrades players can imprint on gear items to transform character abilities in unique and interesting ways. These Aspects are a key part of character progression and builds in Diablo IV, allowing players to improve their favorite character abilities and augment them with new passive effects. However, unlocking and using them can be a tricky process.

Codex of Power Aspects can be applied to most gear items; however, imprinting Aspects on Legendary quality items will replace any existing item abilities. As of writing, there are 115 total Codex of Power Aspects, divided into Defensive, Offensive, Resource, Utility, and Mobility categories, depending on the type of Diablo IV items  passive buffs they grant. Some are class-specific, while others can be used for all classes, giving players plenty of freedom when it comes to picking and choosing Aspects for character builds.