The 456

Biology and Anatomy
The 456 (as referred to by the Human race) are an 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. Their real name remains a mystery. Their bodies natural design consists of three heads, connected to one body via three long necks. Attached to their anatomy is an artificial life support system which is composed of one human child, hard-wired into their bodies. The process apparently does not harm the child, but prevents it from aging, feeling pain and dying. They also remain conscious throughout it. The child has to breathe in the toxic atmosphere required by the 456 through a respirator. However, purpose for this 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
First contact was made by the 456 in 1965 when a new strain of Indonesian flu was projected to kill millions and there were difficulties in coming up with an anti-virus. Then, speaking through radio waves on the signature frequency, 456 came alien voices, claiming to have an anti-virus and that they were prepared to trade with the government of the United Kingdom: the anti-virus for twelve children, and a promise never to return to Earth.

The 456 gave the government a meeting point in the Scottish Highlands. Captain Jack Harkness drove a bus containing twelve orphan children to the location. The 456 said that the children would not be harmed, and in fact "would live forever." Harkness and the armed personnel ushered the children into a great radiant light, and with the exception of one orphan, the 456 took the children and vanished, transmitting the information needed to create the vaccine.

In 2010, the 456 returned to Earth. They first stopped every single child on the planet and used them to convey the message "We are coming." They followed up by secretly using the radio frequency 456 to relay instructions for a purpose built gas-filled chamber to be constructed for them on the top floor of Thames House. On their arrival day they came through the sky in a pillar of fire into the chamber, and began negotiations with John Frobisher, this time asking for 10% of the world's children in exchange for not destroying the human race.

The request was denied and the UK government offered 6200 children; the 456 rejected this offer and used the children of each country to tell the prospective governments the number of children required by the 456. 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 didn't comply, then gave the governments of the world one day to consider.

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.