The 456 ambassador was a member of the 456, who came to Earth in 2009 to extract 10% of its children for the purpose of drug-dealing and usage. The ambassador was the only individual of the species with which humanity had direct contact.
The ambassador had one of the twelve children from 1965 wired into its body. (TV: Children of Earth: Day Four) The 456 race extracted fluids from the brains of human children to create a sensation equivalent to drug intoxication. (TV: Children of Earth: Day Five) The ambassador arrived in a pillar of fire and landed in a specially built tank in Thames House. (TV: Children of Earth: Day Three) It relayed its race's demands to the government and when confronted by Jack Harkness and Ianto Jones, showed the extent of its power by releasing a virus into Thames House that killed all inside. (TV: Children of Earth: Day Three, Children of Earth: Day Four)
Once the government had agreed to hand over the children, the ambassador decided to wipe out the last remnant of the race's last visit to Earth and killed Clement McDonald with a hypersonic signal. (TV: Children of Earth: Day Four) However, this proved to be the ambassador's undoing; Torchwood sent the same signal back at the ambassador, seemingly killing it as bursts of its blood splattered against the inside of its container. However, the contents of the container were removed from the Earth in another pillar of fire before the creature's death could be verified. (TV: Children of Earth: Day Five)