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.
Megatron later summoned his then-medic Knockout to retrieve the saber while he battled a defected Predaking, and after disposing of Predaking by throwing him out of his warship's airlock, presuming him dead (though he later was revealed to have survived) Knockout delivered the sword to him.
He the used it throughout a battle involving the Autobots in space over control of a reconstructed Omega Lock that took place not long after the Predaking fight, which culminated in Megatron's eventual death at the hands of Bumblebee (who was killed by him and quickly brought back to life by the Omega Lock's Cyber-Matter). The Dark Starsaber was dropped back onto Earth.
Robots In Disguise
After the Dark Starsaber was dropped onto Earth, it fell into an unknown part of the ocean (presumably next to Megatron's corpse which also fell into Earth), where it was undisturbed for over a decade until a group of Decepticon scavengers picked up it's signature (though they did not know what the saber was) and landed their ship onto a nearby unmanned Cargo ship. With help of a group of reluctant Minicons, the scavengers took the weapon and quickly began to dispute over who should take possession of it. The bickering was stopped however when the Minicons took off in the scavenger's ship and the arrival of Starscream (Megatron's former second-in-command), who was looking for the escaped Minicons and was pleasantly surprised at the sight of the weapon. He then used it's powers to throw the scavengers into the ocean, despite the sword not thinking Starscream was worthy, and left the Cargo ship using a Ground-Bridge.