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 named so because on their first contact with Earth in 1965, they were transmitting on a frequency of 456. Through unknown technology, they are able to use human children to talk and point, relaying their messaged without revealing themselves. Upon their second arrival they descended into the tank in Thames House in a giant pillar of fire, passing through the roof of the building without any apparent damage to the roof.

History
In 1965 the 456 came to Earth and were given a "gift" of 12 school children by Jack Harkness because they said that they could provide a cure for a virus which would kill a lot of people. Jack implies that with this gift they had promised not to return to Earth, a promise they broke in 2010. The species used the children as part of a composite life support system surgically inserted into their bodies. The 456 returned to Earth in 2010 and heralded their arrival by taking over every child on earth several times to relay messages such as 'we are coming' or 'we are here'. They ordered that a chamber be built at Thames house containing a suitable atmosphere for them before descnding within a pillar of fire into the tank. They were asked to keep the exsistance of their previous dealings with Britain a secret. They demanded that the earth hand over 10 percent of their children to them. The government agreed reluctantly but when Jack Harkness confronted them they declared war.

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.