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The Shakri were supposed myths in Time Lord history, said to be “the pest controllers of the universe”.
The “Shakri” was an automated interface system on the Shakri ship, described by the Eleventh Doctor as a “talking propaganda poster”. He told the Eleventh Doctor that the Shakri lived in “all of time and none” and that they served the Tally. He also told The Doctor that they would send a second wave of cubes to Earth after the first wave had put a third of the population into cardiac arrest. (The Power of Three)
The Doctor explained that the Time Lords were taught about the Shakri when they were young, but believed them to be legends. (The Power of Three) Romana II knew of them as “characters in a nursery rhyme”. (Babblesphere) they existed “in all of time, and none. [they] travel[led] alone and together “.
In the 2010s, the Shakri decided that humans needed to be destroyed before they could colonise space. They sent millions of inert Shakri cubes to Earth, then did nothing with them. After a year, humanity had gotten used to them and taken them into their livesat this point, they were activated for 47 minutes, learning all they could about humanity. After counting down from seven, they then stopped the hearts of anyone close by. The Shakri planned to release a second wave of cubes to kill the remaining humans, but the Eleventh Doctor was able to reverse the effects so that the cubes instead restarted the victims’ hearts. (The Power of Three)
One Shakri ship once visited Trenzalore when an uninterpretable message was heard. The spacecraft was destroyed during the Siege of Trenzalore. (The Time of the Doctor)