An ancient Menthar device that absorbs energy from a passing starship and returns it as lethal radiation, a field of which traps the Enterprise-D at Orelious IX.
Aceton assimilators are energy collectors that the Menthar hid in the debris of Orelious IX during their final battle with the Promellians a thousand years ago. They appear in the Next Generation episode Booby Trap.
The devices draw power from any vessel that comes within range and convert it into radiation directed back at the ship. The more power the victim uses, the faster it is drained and irradiated. The Promellian battle cruiser Cleponji was caught this way and its crew died at their posts.
The Enterprise-D falls into the same trap while exploring the derelict. Geordi La Forge works with a holodeck recreation of the engine designer Leah Brahms, and Picard finally flies the ship out by hand on minimal thrusters before destroying the field.