The Overmind the center of the Zerg Swarm's hive mind society, created by the Xel'Naga as a single consciousness for their experiments on the Zerg,secretly enslaved
by the fallen Xel'Naga Amon, who implanted it with a directive to destroy the Protoss and bound the Zerg to a hive mind.The Overmind becomes aware of Xel'Naga, 
attacking and assimilating its masters.Through this, the Overmind learns of the existence of the Protoss and is determined to assimilate the fellow Xel'Naga-empowered 
species, believing this will result in perfection.To provide the necessary force to overcome the Protoss, the Overmind targets the psionic potential of the Terrans
for assimilation.this is why it assimilated Sarah Kerrigan so as to create a weapon to free the Zerg from the control of it's Dark Master. The Zerg and Protoss clash on 
various Terran worlds, eventually leading to Zeratul assassinating the cerebrate Zasz. Zasz's death momentarily links the minds of the Overmind and Zeratul, allowing 
the Overmind to learn the location of the Protoss homeworld Aiur. The Overmind quickly launches an invasion of Aiur and manifests itself on the planet's surface. 
Following a lengthy and costly campaign on Aiur. later Tassadar harnesses the energies of the dark templar to strike at the Overmind directly, disintegrating it.but 
nobody noticed a small wormhole opening near the overmind during it's final moments .as it gets fragmented ...a big portion of it gets sucked into the wormhole .it 
heads forworod to earth's surface ....as it burns in high like a meteor ...the ramnent dark energies lingering on it gets assimilated with it and turns a dark purple 
psionic energy.but as it burns it gets smaller and smaller .every cell trying hardest to evolve as it lands heavily on one of earths city Kolkata.this is where the 
story begins.

zeratul build

Zerg Resurgence On Earth