The Thrake were a species of memory parasites that the Eleventh Doctor described as "a nasty piece of work". Having the ability to float, Thrake resembled hooded ghosts with long green arms. The call of the Thrake was "wisssaaa".
One Xerxes Memory Ark in the Antrozenus Zone became infested with Thrake looking to feast on the memories of the Xerxes at the heart of the Ark. The Eleventh Doctor, Alice Obiefune, and the Sapling visited the Ark and discovered the Thrake. The Doctor decided to lead the Thrake directly to the interface of the sub-space memoryweb of the Xerxes, allowing the Thrake to feast on the memories of an entire civilisation. However, the sheer amount of memories overloaded the Thrake, killing them. (COMIC: The Memory Feast)