Xel'Naga

From The StarCraft 2 Wiki

The Xel'Naga are the race of a very powerful (or technologically advanced at least) people that created many races, including the Protoss and the Zerg. In reality, the actually guided and sped up the evolution on the two races. The Protoss are their'Firstborn', achieving 'Purity of Form', while their second was the Zerg, who have 'Purity of Essence'

Apparently they are on the way back, and they are not happy with current turn of events.