After the Xerxes were hit by a great plague, Xerxes scientists tried to make a cure for the plague, but only succeeded in creating a disease which was even more fatal. Seeing that the complete extinction of their species was imminent, the Xerxes created for themselves a virtual afterlife; the bodies of individual Xerxes were placed in Memory Arks which created a library of all of the Xerxes' memories. The Arks were then connected via a sub-space memoryweb to form a virtual database that would preserve the legacy of Xerxes civilisation.
For a few billion years, the Memory Arks of the Xerxes drifted through space, not even being encountered by the Doctor.
This changed when the Eleventh Doctor, Alice Obiefune, and the Sapling boarded a Memory Ark in the Antrozenus Zone without even knowing what type of ship it was. After some exploration, they found the body of the Xerxes whose memories were being preserved by the ark and learned that the Thrake were trying to consume the memories. The Doctor let the Thrake feed off of the sub-space memoryweb connecting all of the Xerxes arks, sacrificing a few of the species' memories to overload and kill the Thrake. (COMIC: The Memory Feast)