456 hypersonic signal

The 456 hypersonic signal was a deadly signal utilised by the aliens known as the 456. (TV: Children of Earth: Day Four)

Overview
When emitted, the signal could be heard through monitoring equipment, but at the same time could be sent into a specific target's brain, at least if they had a psychic link to the 456. When used on a human, the signal appeared to induce a fatal brain aneurysm in the target, resulting in bleeding from the nose, mouth and ears and eventually, death. (TV: Children of Earth: Day Four)

When used in the form of a constructive wave against the 456 themselves, the signal had deadly effects. The one known effected subject writhed around in great pain before appearing to violently explode. However, its death was unconfirmed. (TV: Children of Earth: Day Five)

History
At the same time it released a deadly virus into Thames House, the 456 ambassador attacked Clem McDonald with this signal through the psychic link that had been established between the 456 and Clem when he was a child. The signal ultimately resulted in his painful death. (TV: Children of Earth: Day Four)

While attempting to find a way to use wavelengths to fight the 456, Captain Jack Harkness wondered about Clem's death and speculated it was because the link between the 456 and Clem hurt them in some way. After Johnson provided Jack with the signal, Dekker informed him that it was new and had never been sent by the 456 before. Jack decided to turn the signal into a constructive wave and send it back at the 456 using the children of Earth as one giant transmitter. Having the signal turned against them in such a way caused the 456 ambassador to writhe in pain before appearing to violently explode. The ambassador's death was unconfirmed as his remains were removed from Thames House in a pillar of fire, but the attack forced the retreat of the 456 and the end of the crisis. (TV: Children of Earth: Day Five)