Technology of the Week Features
An aceton assimilator remotely drains energy from power sources and releases it as hard radiation. The Menthars used them as a type of mine in their war with the Promellians.
The USS Enterprise encountered a field of aceton assimilators deployed in an asteroid field when they investigated a 1000-year-old derelict Promellian warship. The assimilators remained operational despite their age, and they soon began to drain power from the Enterprise, bombarding the ship with radiation in the process. The assimilators drained enough power to keep the ship from maneuvering out of the asteroid field on impulse or warp propulsion (primarily by inhibiting subspace field formation), and the constant radiation bombardment steadily depleted the ship's shields.
The assimilators were also shown to be capable of absorbing the energy of starship phaser blasts without sustaining any damage. They did not, however, prevent photon torpedoes from eventually being used to destroy the Promellian vessel to keep any other ships from being lured into the trap.
The assimilators did not seem to create any kind of gravity well or other phenomenon to hold a "target" ship in place in the field. While warp engines were unable to generate a subspace field, the ship's maneuvering thrusters were clearly sufficient to get the ship moving relative to the asteroid field, allowing them to just drift clear. It's bizarre that it took three hours for a crew member to come up with the idea (let alone that the crew of the Promellian ship never thought of it, and therefore perished).