Dual Energon Swords

Can we say "Mother!"

Now, Optimus Prime may thoroughly believe his credo of "freedom is the right of all sentient beings", but he tends to revoke that in the case of the Decepticons. Thus, when entering combat, Prime will use his ion blaster, his barrage cannon, and his Dual Energon Swords to their fullest effect. Possessing one on each arm, he charges the swords, also known as battle blades, via a quantum electric generator with a max yield of one thousand degrees Celsius and a burn rate of five kilojoules per second.

This makes it a pretty handy weapon. Especially if you have a preference for going after your enemy's face.

Fiction

Transformers (2007) movie

If in the first movie he had one, and in the second movie he had two, then in the third movie, he...ah...

While escorting the All Spark to Mission City, Prime and the Autobots were set upon by the Decepticons Barricade and Bonecrusher, the latter of whom went to robot mode on the freeway to confront the Autobots. Transforming himself, Optimus engaged Bonecrusher in a short but fierce battle, with the Autobot leader punching Bonecrusher off an overpass with sufficient force to knock out one of his optics. Partially blind and more enraged than usual, Bonecrusher began wildly swinging his tail claw, but Optimus ducked behind a support pole and unsheathed his right Energon Sword. Using it to full effect, Optimus sliced off Bonecrusher's arm and then jammed the sword through the Decepticon's neck, decapitating him. Transformers

|===Revenge of the Fallen movie=== While trying to rescue Sam Witwicky from the Decepticons, Optimus was forced into a three-on-one battle against Megatron, Starscream, and Grindor. After having his mouth plate shattered, Optimus unsheathed the Dual Energon Swords, slicing and dicing his would-be destroyers Starscream and Grindor, relieving the former of his right arm and the latter of his arm, rotors, ribs and head using a combination of the swords and a set of hooks.

Later, after Optimus Prime was resurrected by Matrix of Leadership and combined with the parts of Jetfire, he worked his way through the floating ring of debris surrounding the solar harvester to reach Megatron and The Fallen. The battle between them was short, with Optimus slicing off Megatron's fusion cannon arm with his left Energon Sword before sending him flying through some ruins, and ended with Optimus slicing off The Fallen's chin with the blades and proceeding to tear off his face with The Fallen's own staff. Revenge of the Fallen

Notes

It's very possible (and argueably probable) that these types of weapons aren't limited to Optimus like his ion blaster is. Sideswipe's blades have a similiar lightsaber-like effect that suggests that it not only cuts but also melts through objects just like Prime's swords, as we see in Revenge of the fallen. But the fact that these blades don't even glow, as well as Sideswipe's Battle Bio suggest otherwise.