The Forsaken is a faction of humans who managed to survive Tiberium toxemia at the cost of becoming human-Tiberium hybrids. They were added as a new tech faction in version 0.7.
Very little is known for certain about the Forsaken. The faction seems to have been founded and is largely populated by mutants who managed to escape Nod concentration camps, banding together as a natural means to survive. The Brotherhood is known to have rounded up and even artificially "breeding" mutants for their cyborg program, apparently in favour of the mutants' higher tolerance of pain and lower rejection to cybernetic prostheses.
Viewing themselves as a band of outcasts, the Forsaken bow to no-one and use any means necessary to get what they want. They have been raiding GDI- and GloboTech-protected settlements, even decimating armed convoys. This makes it clear the Forsaken are a force to be reckoned with.