Atraxi

The Atraxi were a species that formed a galactic police force.

Appearance
The Atraxi ships resembled giant eyeballs set within a large crystalline structure much like a snowflake. They were able to travel both in space and in atmosphere. (DW: The Eleventh Hour)

Abilities

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

The Atraxi were able to remotely activate and control electronic media on Earth, allowing them to send a message in every human language, through every available device. They were also able to track 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 were unable to find multi-forms when shapeshifted. The Atraxi also had a weapon capable of destroying the Earth (requiring only 20 minutes to charge) and a force field to seal the Earth for its destruction. The Atraxi ship who came directly to Earth was also capable of projecting holograms. (DW: 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. (DW: 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 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 threatening them unless they left earth. (DW: 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. (DW: The Big Bang, A Good Man Goes to War)

Behind the scenes

 * In cut dialogue from DW: 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 DW: Flesh and Stone.
 * In EDA: Vampire Science the Eighth Doctor mentioned the planet Atraxi 3. It is possible this was the homeworld 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 Doctor had "Atraxian semble seeds" in his pocket, presumably from the Atraxi homeworld.