The 456

Biology
The 456 are large creatures that breath a thick gas poisonous to humans. They have three heads and long necks. Their biological life support seems to consist of having a human child wired into thier bodies to connect then up. after this process the child does not age and does not age although they are still conscious.

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.

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.