Dalek Thay

Dalek Thay was a member of and acted as a strategist to the Cult of Skaro.

Origins
Jast was chosen to be part of the Cult of Skaro along with Dalek Jast, Dalek Caan and their leader, Dalek Sec. The purpose of the cult was to "think as the enemy thinks" and come up with novel and unorthodox methods of furthering the Dalek's cause during and after the Last Great Time War. It was their leader, Dalek Sec, who began using their initiative to defeat enemies. (PROSE: Birth of a Legend) During the Time War the cult escaped the carnage by hiding between realities in a Void ship, along with the Genesis Ark, which contained an army of imprisoned Daleks. (TV: Doomsday)

The Battle of Canary Wharf
Thay was the first Dalek to encounter the Cybermen during the Battle of Canary Wharf in 2007. He was tricked into identifying himself by the Cybermen after both sides refused to "identify first". When the Cybermen proposed an alliance, Thay denied them their request, opened fire on them, and returned to the sphere chamber. Thay aided in the awakening process and fought in the following battles against the Cybermen and Torchwood staff to protect the Genesis ark until it opened. Thay spoke in a slightly low-pitched voice with an unusual nasal croak. (TV: Doomsday)

The Final Experiment
In 1930 in New York City, Thay originally had strong feelings against the Final Experiment, even to the point of countering Dalek Sec's orders, claiming that it was against the Dalek ideology of racial purity, but eventually conceded that it was necessary for survival. To complete the Empire State Building before the gamma strike hit Earth, three of the back plates of Thay's casing were removed and attached to the building's mast, being replaced with iron as the bonded polycarbide of the plates was required to attract the gamma strike. (TV: Daleks in Manhattan)

When the Doctor witnessed Dalek Sec become a human-Dalek and free multiple prisoners, Thay, Caan and the pig slaves tried to recapture them. Failing to do so, Caan ordered the slaves to return to base. Once alone, Caan asked Thay's opinion of Sec. Thay stated the cult was created to follow Sec but confirmed he had doubts. When Sec sent Jast, Caan and the pig slaves to attack Hooverville, Thay stayed with his leader to oversee the assault via a visual link and noted Sec's reaction to Solomon's extermination and refusal to allow the Doctor be killed by Dalek Caan. Thay later demanded an explanation as to why Sec had spared the Doctor, Sec replied that the Doctor was a genius and the future of the Daleks depended on him. During the creation of the human-Dalek race, Thay rebelled against Sec and the Doctor with Jast and Caan by overriding the gene-feed with pure Dalek geneo. When the Doctor discovered this, the cult's remaining pure members revealed their true intentions and threatened to kill the Time Lord if he didn't step away from the controls.

When Sec ordered the cult to stand down. Jast stated that the Doctor was an enemy of the Daleks and Sec was designated likewise. Thay then justified Jast's statement that Sec had lost his authority by stating he was no longer Dalek. After the brief argument, Thay ordered the pig slaves to restrain Dalek Sec and the Doctor while the cult monitored the gamma strike for the final experiment. While the Doctor managed to escape, Sec was incarcerated in chains on a nearby wall. When the gamma radiation finally hit and awakened the army of hybrids, Thay proudly declared the accomplishment to his colleagues. Once fully awakened, the cult informed the new species of their plans and armed them with Dalek weaponry adapted from Tommy guns. As the new soldiers spread under Manhattan using the sewer system, Caan pronounced himself the new commander, and ordered that he be connected to the military computer, despite Sec's protests that the position was reserved for him; Thay simply stated that his former leader was no longer fit.

As Caan transmitted orders to the Dalek-humans, Thay monitored the command links to make sure they functioned consistently. During command, Caan later detected a signal from a sonic device. Thay instantly identified the source without the need of any equipment. However, he was taken aback that the Doctor had survived the numerous death traps he'd walked into. Thay and Jast were then dispatched to the theatre to face the Time Lord. While confronting the Doctor in the theatre, Thay claimed the Time Lord had to die and was about to fire when Sec, now imprisoned with his chains on Thay's Manipulator arm, threw himself in front of Thay's death ray causing his extermination.

After the Doctor explained that the Dalek-humans weren't Daleks and that his Gallifreyan DNA was mixed with their Dalek and human DNA in the "Final Experiment", the Dalek-humans questioned orders and revolted against their Dalek masters. Thay immediately opened fire on the hybrids claiming, "If they will not obey then they must die!" Although Thay and Jast exterminated many Dalek-humans in the process, the resulting fire fight destroyed them both. (TV: Evolution of the Daleks)

Personality
Dalek Thay had the second-lowest-pitched voice of the Cult, with a nasal croak. Thay was the least reserved of the Cult and would often voice his opinion to his leader. He would often taunt his opponents with Dalek ideals and always seemed to want the Dalek race to be "pure", as demonstrated during the Final Experiment. During the experiment, he protested to Sec's bonding with Mr Diagoras and later protested when the Doctor and Sec spoke of how Davros was wrong to strip the Daleks of emotions, and that it would be better to no longer be the "supreme beings". He seemed very loyal to Dalek Sec until his hybridisation, and to Dalek Caan when they revolted against Dalek Sec. He did not hesitate to exterminate anyone who stood in their way, including Sec himself although this was an accident.

Other information
Dalek Thay, as with all Daleks that originated from the Last Great Time War, possessed markings below his eyepiece to identify him. (TV: Doomsday)