Death Squad Dalek

Death Squad Daleks were a variant of bronze Daleks who made up a Dalek Death Squad. The Thirteenth Doctor described the Death Squad Daleks as "the SAS" of Daleks, noting that they were the "enforcers" of the Dalek race's "purity"; their priority was not the invasion of alien planets but the destruction of any mutant or impure Daleks. (TV: Revolution of the Daleks)

Characteristics
Death Squad Daleks appeared externally identical to bronze Dalek drones, but had a different attachment instead of the usual manipulator arm seen on a standard Dalek. Leaders of groups of Death Squad units were identical to the lower ranking members of the Squad, though they had a deeper voice. (TV: Revolution of the Daleks)

Death Squad Daleks were the "enforcers" of the Dalek race. Their main goal was to listen for a Reconnaissance Scout Signal. They would travel through the Time Vortex to the source of the Reconnaissance Scout Signal. If there were impure Daleks, the squad, like all Daleks, made it their duty to exterminate them. If there were no remaining impurities, they would start conquering the planet. (TV: Revolution of the Daleks)

History
In 2021, the clone of a Reconnaissance Dalek and its army of Defence Drones attempted to exterminate the human race on Earth. The Thirteenth Doctor decided that the only means of stopping them was to summon more Daleks to deal with those already on Earth. Broadcasting the Reconnaissance Dalek's Reconnaissance Scout Signal through the Time Vortex, the Doctor was able to draw the attention of a Dalek saucer and the Death Squad Daleks aboard it.

Upon arriving at Earth, the Death Squad Daleks detected the "impure" Defence Drones on the planet below and thousands of Daleks were dispatched from the ship across the planet to exterminate them. With her plan working, the Doctor dispatched Jack Harkness, Ryan Sinclair and Graham O'Brien to destroy the Death Squad's ship, although she ordered them not to detonate their bombs until they were sure that the Death Squad Daleks had destroyed all of the other Daleks on Earth, and the trio had escaped the ship. Teleporting aboard using Jack Harkness' vortex manipulator, the three men spread out and began planting explosives supplied by Jack all over the ship, avoiding the command centre where most of the Daleks were located. However, Jack Robertson betrayed the group and was transported up to the ship, where Jack witnessed him informing the Dalek leader of the Doctor's presence and the trap she had laid for them. The cloned Reconnaissance Dalek, the last surviving Defence Drone, then teleported aboard the ship and attempted to bargain for its life to the other Daleks, but was nonetheless exterminated for its impurity.

Jack warned the Doctor, who knew that destroying the ship would not stop the Daleks attacking Earth. As the Daleks detected the Doctor's location, Jack, Graham and Ryan grabbed Robertson and fled as they were detected, attacked and cornered by the Daleks. Yasmin Khan signaled Jack, who teleported the group out. As they teleported out, Ryan detonated the bombs, destroying the Dalek ship and all of the Daleks still aboard it.

On Earth, the Doctor's spare TARDIS appeared in the skies, taking on the form of her regular TARDIS thanks to its chameleon circuit. Broadcasting a holographic image of herself, the Doctor lured the thousands of Death Squad Daleks still on Earth inside. The Doctor revealed her deception, but the Daleks were sure that they would find and exterminate her, using the TARDIS that they now had control over. However, the Doctor revealed that she had programed the TARDIS to fold in on itself and send itself to the heart of the Void, breaking the TARDIS apart and destroying the Daleks in the process. The Doctor tauntingly thanked the Daleks for their help and promised that she would escape from them every time. The TARDIS then imploded and teleported into the Void, destroying the remaining Death Squad Daleks. (TV: Revolution of the Daleks)