Energon mace

Although the fusion cannon is Megatron's more notable weapon, the leader of the Decepticons has many others. Among the more infamous weapons of his is the Energon mace.

On Earth, he first used it when fighting Optimus Prime on a particular dam.

More than Meets the Eye
When Megatron led the Decepticons on a raid of Sherman Dam, Megatron and Optimus Prime battled on the top of the dam. As the Autobot leader brought out his energon-axe, Megatron revealed his energon mace. After some obligatory trash talking, Megatron and Prime proceeded to thrash each other. However, Megatron only won when Prime's attention was turned to Spike Witwicky calling for help, allowing Megatron to knock Prime off the dam, and fly away with the Decepticons.