The Dark Star Saber is a weapon from the Transformers Prime.
The Dark Star Saber is a sword made from pure Dark Energon. Intended to be a dark counter part of the Star Saber itself, this blade possesses all the powers of the original blade and is even stronger than its counterpart.
Megatron, after securing the arm of a Dead Prime used the Forge of Solus Prime to craft a counterpart to the Star Saber out of a huge chunk of Dark Energon. Megatron first brought the blade into action after the decoding of the next Iacon relic in Egypt, and used it to make fools out of Arcee and Smokescreen. The waves of energy it created threw Smokescreen against a pyramid. When Optimus Prime arrived with his own Star Saber, the two engaged in battle, but the Dark Star Saber would prove to be stronger, breaking Optimus's sword.
He took the sword with him to Cybertron to find the Omega Lock. He got the opportunity to use it when the Autobots showed up, and Optimus attacked him with the reforged Star Saber. It also proved capable of destroying the Spark Extractor when Smokescreen used the device against his men.
After Megatron began using the Omega Lock to cyberform] Earth, Optimus charged at him with the Star Saber. Megatron managed to counter his first blow using the Dark Star Saber, only for Optimus to slice his sword arm off at the elbow. Both sword and arm were subsequently caught in the explosion when Optimus destroyed the Omega Lock.