The Energon Drain is a massive Decepticon weapon constructed in an asteroid field. Presumably it drains energon from things, though it appears to work pretty well for blowing things up too. Megatron believes it will make the Decepticons unstoppable.
The Autobots found out about the Energon Drain and used the debris in the asteroid field to build a Pulsar Dish. Despite the Decepticons mounting a resistance, the Autobots managed to complete their counter-measure in time and destroy the Energon Drain.