The 456

"You yielded in the past. You shall do so again."

- 456 Ambassador to Captain Jack Harkness

The 456 (as Humans called them, a designation which they appeared to prefer) were an alien race who secretly negotiated with representatives of the Human race in 1965 and again in 2009.

Biology and Anatomy
Very little is know about the 456's biology and anatomy, as humanity has only had contact with one member of the species, and that was on Earth, the individual remained within a very opaque gas chamber during that time, with minimal visibility. It was later reveled that the individual may have been under the influence of a recreational drug the entire the time, the effects of which are not fully known. Though they could develop a symbiotic relationship with Humans, the 456 thrived on a gas made of cyanide, hydrogen chloride and fluoride.

The natural design of their body consisted of three "heads", connected to one body via three long necks. They have the ability to emit a viscous yellow fluid in moments of high frustration. Attached to their anatomy is an artificial life support system, which is at times composed of one human child, wired into their bodies. During the process - according to the Ambassador - the child feels no pain and lives long beyond its natural lifespan. However, it is conscious throughout the experience and has to breathe in the toxic atmosphere through a respirator. The reason for this is that the body of a human child produces certain chemicals that stimulate feelings of pleasure in the 456, much like a drug. This could imply an addiction to the pleasure human children could offer them, leading to a further need for more children. (TW: Children of Earth: Day Three, TW: Children of Earth: Day Four, TW: Children of Earth: Day Five)

Technology
The 456 were 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 appeared to fluently speak and understand English though this may or may not have been through the use of a translator.

They also appeared to be far more advanced than the human race, having the chemical and biological knowledge to provide a range of viruses and anti-viruses to Humans 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 their ship(s) still remaining hidden from advanced radar and satellites.

They could also manipulate the minds of millions of children at will, as displayed when they came to Earth in 2009, when they forced all children globally on Earth to chant "We are coming" for a matter of seconds. The transmissions carried on more frequently, until it got so sophisticated that they had each country's children relaying a different message to their own governments. (TW: Day One, Day Four)

1965 contact
First contact was made by the 456 by radio on that wavelength, hence their designation. In 1965, they made a deal with a small group of Humans to give to them the antidote for a new strain of Indonesian flu projected (by the 456 or the Humans) to kill millions. The 456 the anti-virus in exchange for twelve children. 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 the light and with the exception of one orphan, the 456 took the children and vanished. (TW: Children of Earth: Day Four) 

2009 contact
In September 2010, the 456 sent a message through every single child on the planet. Then they verbally conveyed the message (in English): "We are coming." (TW: Children of Earth: Day One) They followed up by secretly using the radio frequency to relay instructions to Dekker 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, an ambassador transported itself from 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: Children of Earth: 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 respective 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 demonstration of what the people of Earth would receive if they didn't comply. This killed everyone in Thames House including Ianto Jones and Jack Harkness. The 456 then gave the governments of the world one day to consider. (TW: Day Four)

The government thus continued with its operations to round up 10% of the UK's children and hand them over to the 456. Fortunately before the 456 could take their prize Jack Harkness was able to turn back on the 456 the radio wave which had killed Clement and hence also killed them, thus saving the Earth. This however would come at the cost of Harkness' own grandson. It is unknown if any of the 456 were killed, alltough the individual at Thames House may have exploded, but the contents of the gas chamber were removed in another "great pillar of fire" before it could be investigated. (TW: Day Five)

Behind the Scenes
To be added