The Twenty were twenty citizens of Metropolis, who were modified by Brainiac while the city was shrunken. Their intelligence was raised, and they were given psionic powers. Each of the twenty seemingly disappeared shortly after the attack, beginning new lives for themselves.
One of the Twenty, a woman known only as the H.I.V.E. Queen, kidnapped the others and started an organization called H.I.V.E., or "Holistically Integrated Viral Equality", whose purpose was to link everyone together in a collective consciousness, in preparation for Brainiac's return. (Superman: "Psi War")