Zhartuus was a Cerebrate that evolved seperately from the others of it's strain due to a genetic glitch caused from the severing of the psychic link between the Overmind and the Xel'Naga. Zhartuus developed a psychic immunity to the dominating thrall of the Overmind, but continued to play along with it's commands. Although it obeyed the Overmind for countless milennia, it still clung tightly to one thought; "The time shall come when I shall rule the swarm, for I am the mastermind". Upon the death of the Overmind, Zhartuus merged with other Cerebrates to form a brain-like entity of indeterminate age.
From that time forward, Zhartuus has referred to itself as "Mastermind", and it's followers were termed the "Dark Swarm".
Interestingly, Zhartuus is not thought to have been manipulated by the "Dark Voice", and therefore seemingly carries what the Overmind did not... Free will. Although Zhartuus was created under the indoctrination of the Overmind, and therefore bears a will to consume other races to grow stronger, there have been instances where Zhartuus has aided other factions, for a price.