The 456

The 456 (pronounced four-five-six, not four hundred and fifty six) secretly negotiated with the British government in 1965, and again in 2009. Their designation was given to them by the Human race based on the radio wavelength they favoured, and they took the name as their own in their dealings with Humans rather than offering the actual name of their species, if any existed.

Biology
Very little is known about the 456's biology and anatomy, as Humanity only had contact with one member of the species, and that was on Earth, during which the individual remained within a very opaque gas chamber, with minimal visibility. The natural design of their body consisted of three "heads", connected to one body via three long necks, possibly a conjoined triplet specie. It resembled a large, plucked, three headed swan. (TW: Children of Earth: Day Four) The 456 thrived in an atmosphere made up of 25% nitrosyl chloride, 22% hydrogen chloride, 20% nitrogen, 12% fluorine, 9% hydrogen cyanide, 6% acetone, and 6% phosgene. (TW: Children of Earth: Day Two)

It was later revealed that the individual may have been under the influence of a recreational drug the entire time, the effects of which are not fully known. The 456 Ambassador would emit a viscous green fluid in moments of high frustration; this may be a side effect of either their use of recreational drugs or a natural response. (TW: Children of Earth: Day Three) Their blood was red in color, though this may have been from the child. (TW: Children of Earth: Day Five)

Technology
The 456 were known to have technological systems which could match the systems of the Human race from 1965 onwards, such as radio telecommunications. They spoke English fluently and perfectly understood it; 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, indeed far more advanced then most other species as well, having the chemical and biological knowledge to provide a range of viruses and anti-viruses to Humans. (TW: Children of Earth: Day Four) The 456 possessed and utilized the ability to use Human children as a source of drugs. They would forcibly attach a child to their body using a life support system, which would allow the child's chemicals to stimulate feelings of pleasure in the 456. During the process - according to the Ambassador - the child felt no pain and lived long beyond its natural lifespan. However, it was conscious throughout the experience and had to use a respirator to survive in the toxic atmosphere. (TW: Children of Earth: Day Three, TW: Children of Earth: Day Four, TW: Children of Earth: Day Five)

The 456 arrived on Earth in what was described as a "pillar of fire", allowing them to teleport into a specially made tank, despite it being within a building. UNIT believed that their base of operations was located somewhere in orbit of Earth, but they were unable to find it. (TW: Children of Earth: Day Three) In 1965 the 456 took the children in a similar way, teleporting them away into a flash of light. (TW: Children of Earth: Day Four)

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 became more and more sophisticated with each sending, to the point where the 456 could manipulate each country's children into relaying a different message to their various governments in various languages. Using this connection they could also give off a fatal frequency, which they used to kill Clement MacDonald. (TW: Children of Earth: Day One, Children of Earth: Day Four) However, if reversed, the frequency was deadly to the 456, causing their bodies to exhibit extreme pain before violently exploding in a manner similar to vacuum decompression. (TW: Children of Earth: Day Five)

First contact
The 456 first made contact by radio on the wavelength sometime during or prior to 1965. The 456 had learned English and knew something of Earth culture.

In 1965, Andrew Staines, Ellen Hunt, Michael Sanders, and Jack Harkness implemented a deal with the unseen aliens. In exchange for twelve Human children, the 456 would give them an anti-virus for a mutated strain of Spanish influenza, which would otherwise (according to the aliens, whose projections were backed up by British analysts) have killed millions. The 456 claimed that the children would not be harmed, and in fact "would live forever" and that they (the 456) would not return to Earth. (TW: Children of Earth: Day Three, TW: Children of Earth: Day Four)


 * What governmental bodies the first three people represented remains unclear.

Twelve orphans from the Holly Tree Lodge orphanage were taken under the pretence that they were to be taken to another orphanage called Harbour Heights. The children were brought to a location in Scotland, where they were ushered into a white light. For unknown reasons, the 456 decided to leave behind one orphan, Clem MacDonald, who ran off alone into the night. Torchwood Three later speculated that they weren't interested in him because he was hitting puberty (TW: Children of Earth: Day Four)

Initial contact
In September 2009, the 456 sent a message through every single child on the planet: "We are coming." (TW: Children of Earth: Day One) They followed up by secretly using the radio frequency to relay detailed instructions to Dekker for a purpose-built gas-filled chamber to be constructed for them on Floor 13, the top level of Thames House, MI5's headquarters in London and the most secure building in Britain. Technicians set about constructing the containment facility before the aliens' arrival. (TW: Children of Earth: Day Two)



Arrival
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, who Prime Minister Brian Green had managed to have appointed as informal ambassador. They insisted on addressing all the major powers, but Frobisher was able to convince them to keep quiet about the earlier dealings with the British government in 1965. The 456 asked for a "gift" of 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 6700 children of Failed Asylum Seekers; 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 almost everyone in Thames House, including Ianto Jones and (temporarily) Jack Harkness. The 456 then gave the governments of the world one day to consider. (TW: Children of Earth: Day Four)

Resolution of the crisis
The government thus continued with its operations to round up the lowest performing 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. Doing so either killed or otherwise repelled the 456, thus saving the Earth. It is unknown if any of the other 456s were killed. 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. To implement this plan, Harkness reluctantly but knowingly killed his own grandson. (TW: Children of Earth: Day Five)

Behind the scenes

 * Humans used the name "456" to refer both to the Ambassador and to the species generally.
 * The loud screech made by the aliens was previously used by The Swarm in the Planet of the Dead.
 * The 456's heads are are similar to the Star Whale's appendages in the Doctor Who episode, The Beast Below, though it is notable that said episode aired after Children Of Earth and thus was possibly based on the 456's appearance.