The "Supreme Being" was a creature created by the zerg on Char through the use of the xel'naga artifact, the Argus Stone. It reached maturity within a chrysalis.
Xel'naga lore predicted that a catastrophic war would precede the birth of the legendary being, and only one race would emerge triumphant.
Terran and protoss forces, which had been seeking the Argus Stone, attempted to destroy its chrysalis, but were defeated by zerg forces.
The "Supreme Being" bears resemblance to Sarah Kerrigan—both develop within a chrysalis, are highly prized assets of the Swarm during the Great War, and are figures of note within xel'naga prophecy.