Invisibles are a race spawned in outer space, in the darkness between the stars, the offspring of a "cosmic Satyr". This almost Lovecraftian description is apt since in physical form these creatures resemble arthropods with a singlle eye, feelers, cilia and many legs.
Parasitic, the bond physically and permanently with an intelligent host, merging their physical form with their host. This process can fail, which warps the body of the host or drives them insane.
In the original series episode "The Invisibles" these monstrous entities have a geopolitical conspiracy to take over the Earth.