Cell 114

Cell 114 was the official designation for a group of sleeper agents that Jack Harkness was aware off. Very little was known about them, since they didn't usually left survivors

A specific group of sleeper agents was in the process of infiltrating Cardiff on Earth until Torchwood 3 became aware of them in 2008 trough unusual behaviour from one of the agents, Beth Halloran. At that point they activated, and tried to set in motion a plan to explode several nuclear missiles stored in an old mine near the city. This attempt was thwarted, but it is unknown if more sleeper agents are active on Earth

methods
They tended to infiltrate planets, first with an advanced guard, sometimes watching for years, untill they knew all they wanted to know about the species and were ready to take over.

They adapted their bodies to look human, at least partially trough projecting false sensor images. They also projected a forcefield less than a nanometer above their arm for protection.

Their main resource seems to be a pod located in the left arm (at least when mimicking humans), which contained the pod collecting data and capable of transmitting it to the main invasion fleet. It also contained at least one small bomb for sabotage or self-destruct. Furthermore the arm could be transformed into a sword-like appendage for close combat.heir skin as well as false sensor images of normal anatomy. (TW: Sleeper)