Primordial Soul

Primordial Soul is a Legendary Source for the  in Diablo III, added in patch 2.4.0. It requires character level 68 to drop.

The unique affix works best for any build with multiple damage types, especially when combined with Tal Rasha's Elements. With a total of 10% damage boost per stack, it grants a total of up to 40% bonus.

Trivia

 * It is the first item in game with a special affix specifically enhancing a passive (rather than active) skill.