The Zerg look like monsters , and are all controlled by a very powerful Zerg called the Overmind .
The cerebrate 's death results in its forces running amok through the Zerg hives , but briefly links the minds of Zeratul and the Zerg Overmind , allowing the Overmind to learn the location of the Protoss homeworld Aiur , which it has been seeking for millennia . The Zerg promptly invade and despite heavy Protoss resistance , the Overmind is able to embed itself into the crust of the planet .