Death Squad Dalek

Death Squad Daleks were a variant of Bronze Daleks who made up the Dalek Death Squad. The Thirteenth Doctor described the Squad 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.

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 looked exactly like the lower ranking members of the squad, though they had a deeper voice. (TV: Revolution of the Daleks)

Primary Goal
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, with the clone of the Recon Scout Dalek and his army attempting to exterminate the human race on Earth, the Thirteenth Doctor decided that the only chance was the nuclear option of summoning more Daleks to deal with the ones already on Earth. Broadcasting the Reconnaissance Scout Signal throughout 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 Daleks detected the impure Daleks 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 blow the ship up, although she ordered them not to detonate before they were sure that the SAS Daleks had destroyed all of the other Daleks on Earth and not before the group got out. 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 center 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 Dalek teleported aboard the ship and made his case to the other Daleks, but was nonetheless exterminated for his impurity.

Jack warned the Doctor who knew that destroying the ship would not stop the Daleks still marauding through the Earth's skies. 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 the chameleon circuit. Broadcasting a holographic image of herself, the Doctor lured the thousands of Death Squad Daleks marauding through the planet's skies inside. The Doctor revealed her deception, but the Daleks were sure that they would find and exterminate her using the TARDIS 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)