Es'Cartress of the Tactires was a cranial parasite from Callufrax Minor and the last of the Tactires.
When Callufrax Minor was stolen from time by the Daleks and pulled into the Medusa Cascade, Es'Cartress somehow made it to the Crucible, unnoticed by both the Daleks and the allies of the Doctor. When the Daleks were defeated, he was also able to escape into the Doctor's TARDIS. When the Doctor was alone, Es'Cartress attacked, attempting to take over his mind. The Doctor's link with the TARDIS saved him and in an attempt to fight back, they were all pulled into the TARDIS' Matrix.
Within the Matrix, the three minds were able to form a world, creating a museum based on the Doctor's past. Es'Cartress took the form of the Meta-Crisis Tenth Doctor and called himself the Valeyard, while the TARDIS initially took the form of Martha Jones. Es'Cartress tried to steal the Doctor's memories, while the TARDIS helped the Doctor regain them. The Doctor was eventually able to regain all his memories and confront Es'Cartress. Using the memories and will-power from his previous regenerations, the Doctor was able to take back the Matrix and kill Es'Cartress.
When the Doctor woke up again, he kicked the disgusting parasite out of the TARDIS. (IDW: The Forgotten)