The Kraal androids were created by Styggron as part of the plan to conquer Earth. Physically, they were identical duplicates of the humans in whose image they were created, had most of the memories of their subjects, were nearly indestructible and completely loyal to the Kraals.
Other androids were made for maintenance work which resembled humanoids in white jumpsuits and space helmets and had guns built into their hands, but lacked human faces.
Android duplicates were made of Harry Sullivan, RSM Benton, Colonel Faraday, Corporal Adams, other UNIT personnel, the Devesham villagers (including Morgan), and the Fourth Doctor and Sarah Jane Smith after they arrived on Oseidon.
Styggron planned to use them to replace key personnel on Earth, then spread a deadly virus to infect and kill all of the humans, thus leaving the planet free for the Kraals to colonize. The Doctor stopped the androids with the Space Defence Station's radar dish. (DW: The Android Invasion)
What appeared to be the remains of a Kraal android were held in a cell on the Megara's spaceship. (DW: The Stones of Blood)
SenéNet acquired one of the Kraal androids. (PDA: Business Unusual)