Micro machine (The Heralds of Destruction)

UNIT asked for the Third Doctor's help to deal with micro machines.

The micro machines were able to reunite themselves to form bigger robots. The Brigadier tried to explode them, but they were able to reconstruct themselves.

The micro machines shared a hive mind, and when they attacked someone, the person was fully "colonized", and wasn't able to move. (COMIC: The Heralds of Destruction)