Atraxi

The Atraxi were a sophisticated alien police race, which formed a galactic police force. They were very efficent with their work, tracking down any prisoners who escaped their facilities and executing them. However, they were not the most intelligent of aliens; one failed to notice overloading technology in Leadworth while scanning for Prisoner Zero.

Biology
The appearance of the Atraxi themselves is impossible to know, since they were never actually seen outside of their giant eyeball, which was capable of seeing and speaking. The Atraxi ships had a rather large crystalline structure, much like a snowflake. They were able to travel both in space and in atmosphere. (TV: The Eleventh Hour)

Abilities

 * How many of these traits are technological and how many are natural is unknown.

The Atraxi could remotely activate and control electronic media on Earth, allowing them to send a message in every human language, through every available device. They tracked the media, pinpointing a world-wide computer virus to its origin and learn the planet's history. Though capable of scanning the planet on a worldwide scale, they did not find multi-forms when shapeshifted. The Atraxi fleet was capable of incinerating the Earth (requiring twenty minutes to charge) and a force field to seal the Earth for its destruction. The Atraxi ship which came directly to Earth projected holograms. (TV: The Eleventh Hour)

History
In 102, the Atraxi were among the races who joined the Alliance. They came to Stonehenge and helped imprison the Eleventh Doctor in the Pandorica in order to save the universe. (TV: The Pandorica Opens)

The Atraxi ran an extradimensional prison which contained Prisoner Zero. When Prisoner Zero escaped through a crack in space in 1996, they tried to find it. Twelve years later, they were able to trace it to Earth. They threatened that "Prisoner Zero will vacate the human residence or the human residence will be incinerated".

Though the Eleventh Doctor tried to attract their attention by activating alien technology, his sonic screwdriver burnt out before they noticed. Eventually, he created a computer virus/message and got Jeff Angelo and a team of geniuses to send the message worldwide. The message was '0'. The Atraxi tracked the virus to a mobile phone held by the Doctor. When the Doctor and Amy Pond trapped Prisoner Zero in its own form, the Atraxi were able to find and contain it. Though the Atraxi attempted to leave at this point, the Doctor called them back for breaking Article 57 of the Shadow Proclamation, showing them his past incarnations' victories and telling them to never return to Earth. (TV: The Eleventh Hour)

In the post-Big Bang Two universe, the Atraxi were still called back to Earth and scolded by the Eleventh Doctor. As the cracks were closed, it's not clear if the same reasoning for threatening them occurred. (TV: The Big Bang, A Good Man Goes to War)

Behind the scenes

 * In cut dialogue from TV: The Girl Who Waited, Amy was to have been offered 'Atraxi cuisine'.
 * It is thought that an Atraxi eye watched River Song on a computer screen in TV: Flesh and Stone.
 * In PROSE: Vampire Science, the Eighth Doctor mentioned the planet Atraxi 3. It is possible this was the home world of the Atraxi.
 * In the original script for The Eleventh Hour, the Doctor referred to the Atraxi as a "level 20 civilisation."
 * In Scream of the Shalka, the Ninth had "Atraxian semble seeds" in his pocket, presumably from the Atraxi homeworld.