Energon mace

The energon mace is a weapon in the Generation 1 continuity family.

Verity has nothing on Megatron.

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.

Strangely, he only seems to use it when he is fighting Optimus Prime on a particular dam.

Fiction

Generation 1 cartoon

When Megatron lead 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. More Than Meets the Eye, Part 2

Dreamwave continuity

When Optimus Prime and Megatron were fighting in the bowels of Cybertron, Megatron breached the casing on the Matrix, causing the two Transformers to see visions of the future. Among them were Prime and Megatron fighting with their respective Energon weapons on Earth. The War Within, part 5

From Teletraan I: The Transformers Wiki, a Wikia wiki.