The 456

Biology and Anatomy
An unknown race that live in a toxic environment which air is poisonous to the human nervous system, with it being a compound of various well known chemicals such as cyanide and fluoride to name a few. Their bodies natural design consists of three heads, connected to one body via three long necks, and attached to them is an artificial life support system is composed of one human child hard-wired into their bodies, the process apparently does not harm the child, but prevents them from aging, feeling pain or dying, and they remain conscious throughout, however the nature and reason for the children in this system is unknown at this point

Technology
The 456 are known to have technological systems which meet the systems of the human race from 1965 onwards, such as radio telecommunications, and some chemical knowledge compatible to those of human standards.

They also are far more advanced, they have the chemical and biological knowledge to provide a range of viruses and anti-viruses, they can wire a human child into their own life-support systems, thus living off it, transport themselves across space and through matter with little effort in what can be described as a "great pillar of fire", while still remaining hidden from advanced radar and satellites. The 456 can also manipulate the minds of millions of people at will, as displayed when they came to Earth in 2009, forcing all children globally to recite "We are coming" for a matter of seconds, this carried on more frequently, till it got so sophisticated they had each countries children relaying a different message to the government.

History
Little is known about the 456 before 1965 AD in the UK.

Indonesian flu' was set to put the death rates up by thousands if not millions and Earth was unable to find an anti-virus. Then, speaking through the radio waves on the signature frequency 456 came alien voices, claiming to have an anti-virus and were prepared to go into trade with the government of the UK in, the anti-virus for 12 children, and the promise never to return to Earth.

In the highlands of Scotland, Captain Jack Harkness and members of the armed forces met at an agreed destination point, with a bus full of 12 orphans, supposedly en route to a new orphanage in Scotland, with the promise the children would not be harmed, Harkness and the armed personnel ushered the children into a great radiant light, with the exception of one orphan, when the 456 received the children they vanished, and started to send the components for an anti-virus over the radio.

While Jack felt at the time it seemed like a fair if morbid deal because of what it yeilded Gwen pointed out it sounded more like a protection racket comparing the 456 to nothing better that interplanatary thugs.

In 2009, the 456 returned over a period of two days, they firstly stopped every single child on the planet and used them to convey the message "We are coming", and then using the radio frequency they relayed instructions for a purpose built gas chamber to be constructed for them on the top floor of Thames House, and on the arrival day they came through the sky in a pillar of fire into the chamber, and began negotiations with John Frobisher for more children, 10% of the worlds children to be precise.

The request was denied and a number of 62 was offered, the 456 rejected this offer and used each countries children to tell the prospective governments how many is needed from them. The arrival of Jack Harkness led to an argument with the 456, who then released a poison compound into the air of Thames House as a a demonstration of what the people of Earth would receive if they don't comply and declare war on the 456, then gave the governments of the world one day to consider.

TO BE UPDATED

Rumours

 * It's possible that the 456 are actually Macra, due to breathing gases that are poisonous to humans and the only visible body parts so far have been claws. In Gridlock The Doctor stated that the Macra used to use humans to build their empires for them, so wanting to take the children may be for that.
 * Some believe the 456 to be the same aliens who kidnapped Gray back on Jack's homeworld; if you consider what they do to children and why Clem was spared it was most likely Gray was spared too and had to witness what happened to the other children and other unknown horrors, leading his mind to snap.
 * Early thoughts were that the 456 where the masters of the Alien sleeper agents from Sleeper.