As mentioned in the blue post:
New Talent: Lightning Spark – Chain Lightning marks enemies with Lightning Spark and has a chance to make your next Flame Shock become Tremor Shock. Tremor Shock: Instantly shocks up to 6 Lightning Spark targets with concussive Force, causing Nature damage and applying Flame Shock.
Where is it? I’ve been looking up and down both the Enhancement tree, class tree, hero tree and it’s nowhere to be seen.