Cyber-Unit Ten Six Six was a Cyberman sent by the Cyber-Leader to (along with Unit Ten Six Five) investigate the Sphere Chamber in Torchwood One, resulting in the first confrontation with the Daleks during the Battle of Canary Wharf.
This Cyberman was a member of the contingent which had invaded the Torchwood Tower, the advance guard of of the force of five million Cybermen of a parallel Earth, which had crossed from their parallel Earth to the Earth of the Doctor's universe. Cyber-Units Ten Six Six and Ten Six Five were sent by the Cyber-Leader to investigate unknown technology within the Sphere Chamber which was detected via the Cybermen's scans. Outside the Sphere Chamber, the two Cybermen were met by Dalek Thay, a member of the Cult of Skaro quartet who had itself been sent by Dalek Sec to investigate, the Daleks having learned of a second species invading Earth through a Torchwood employee. Opening a visual link to the Cyber-Leader as Thay did the same for his fellow Daleks, Thay was tricked into identifying itself by the Cybermen after both sides refused to "identify first". When Thay declined their proposal of an alliance, the Cybermen, regarding the Dalek as a "hostile element", moved to delete it. Opening fire on the Dalek, their weapons soon proved ineffective as the Dalek immediately fired back, exterminating both Cybermen and starting a war between the Daleks and Cybermen. (TV: Doomsday)