This has been removed from Heroes of the Storm. 
Tier 3 (Hero Level 7)
Increases Runic Blast's damage by 5% every time you get a Hero takedown, up to 20%. These bonuses are lost upon dying.
Patch changes[]
(Patch September 13, 2016) Note: An icon has been added to the buff bar for the Dark Descent Talent, and the icon’s tooltip will track Runic Blast damage bonuses earned via Hero Takedowns.