The Immobilizer was an experimental Decepticon weapon that was being transported to Megatron on board the Harbinger, which was shot down over earth. It causes instant stasis lock, though the victim remains fully concious. Airachnid found it, and froze Bulkhead and Optimus Prime. Bumblebee pretended to be frozen, but only to distract Airachnid. The Immobilizer was brought back to the Autobot base, where it remained there until the Autobots needed to go to Cybertron to recover the Omega Keys. When they failed, the Decepticons recovered the Immobilizer.