Slitheen

The Slitheen are calcium-formed aliens from the planet Raxacoricofallapatorius. They first appear in the Season 2 finale as part of a London invasion spearheaded by Omega.

Their most distinguishable technology is their compression field, which allows them to disguise themselves as humans by squeezing themselves into their skins. This is done using the compression field collars on their necks. When unzipped, pent up energy is released from the zip in the form of a blue light.

The Slitheen are large bulky humanoids with large arms and powerful long claws. They're usually a yellowish green, but sometimes they're a much darker green.

Despite their large bulk, the Slitheen are very agile and can easily overpower a human. They also have a brilliant sense of smell.

Due to their biochemistry, the Slitheen are vulnerable to acetic acid and react explosively to it.

The Slitheen in the Season 2 finale are sent by Omega to kill all the humans. However, the Doctors and their friends are able to stop them. The Eleventh Doctor fights some of the Slitheen off with a Dreadnought when they refuse to leave, whilst the Tenth Doctor, Sclar, Skaldak and Ood Sigma defeat most of the Slitheen with a giant squirt gun filled with vinegar. Once Omega makes his escape, the remaining Slitheen also flee.