The 456

The 456 where a race of aliens who invaded Earth Twice once in 1965 and once in 2010.

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. (TW: Day Three and Day Four)

Technology
The 456 where known to have technological systems which could 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 where far more advanced than the human race they had the chemical and biological knowledge to provide a range of viruses and anti-viruses. They could 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 could have been described as a "great pillar of fire", while still remaining hidden from advanced radar and satellites. Plus They could also manipulate the minds of millions of people at will, as displayed when they came to Earth in 2010, when they forced all children globally on Earth to chant "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 there own governments. (TW: Day One, Day Four)

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, they claimed to have an anti-virus to cure this and that they were prepared to trade the anti-virus for twelve children, and a promise never to return to Earth. The UK Government did agreed to do so. (TW: Day Four)

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 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. (TW: Day Four)

In 2010, the 456 returned to Earth. Thus not agreeing on there promise. They first stopped every single child on the planet and used them to convey the message "We are coming." (TW: Day One)

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. (TW: Day Two)

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. (TW: Day Three)

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. This killed every one in Thames House including Ianto Jones and Jack Harkness. then gave the governments of the world one day to consider. (TW: Day Four)

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.