The Kraken are creatures at the bottom of the moons ocean that are trying to make more of there race, so they can make the moon there home, but they were never ment to have existed and the only reason they did was because the mystery, who The doctor still hasn`t met yet, made them to try and change time. Why the Mystery did this is unknown as of now, but when The doctor will meet her all will be revieald.

The Kraken are also the same race as the mythocal creatures known as the Kraken, who acording to folklore used to destory boats and kill sailors. But it turns out there a smart race and they went to earth when The Doctor, Kerry and the CIA agents 2 of the main ones (The Big Black ones/the leaders) back to earth, while killing the others because they had to.