The 456

Biology
The 456 appear only to survive within a tank of thick poisonous gas, because of this they cannot be seen other that a pair of claw-like arms. In the trailer for Day Four we see a green like body. They occasionally spit a green slime on the front of their tank.

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. 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 human to build their empires for them, so wanting to take the children may be for that.