Dalek (The Power of the Doctor)

A Dalek betrayed their kind during the events of the Master's Dalek Plan. It believed that the creation of the Daleks was meant to protect the Kaled race, but that the Daleks had fallen too far from that purpose. It contacted the Thirteenth Doctor to reveal their plans, unaware that it was being manipulated by the other Daleks to bring the Doctor to them. Upon arriving, the Dalek allowed the Doctor to copy information from their casing before three other Daleks, including the commander, arrived and killed the traitor.

After the Dalek mutant inside had died, the Daleks imprisoned the Doctor in its casing and took her to 1916 to meet with. (TV: The Power of the Doctor)

Behind the scenes
In the original script, the Spy Dalek identified itself as an elite biological combat engineer whose mission was to devise strategies to ameliorate any Dalek defeat in battle, its work resulting in a genetic code which was to activate and self-execute Dalek creatures in the event of a threat to Dalek supremacy in battle, causing their casings to flood with toxic radiation and detonate, fatally damaging all enemies within a broad radius.