The Key to Vector Sigma was an ancient device that was connected to Vector Sigma and allowed a Transformer to activate the ancient intelligence. It appeared as a small remote with an overdeveloped power circuit and had the ability to control the robotic Centurion droids that remained in the lower levels of Cybertron and defended Vector Sigma.
The Transformers cartoonBy the 20th century, the Key was present in the hands of Alpha Trion on Cybertron until his home was attacked by the Decepticons who gave the key to Megatron in his quest to imbue five cars with intelligence. Megatron would face some resistance from the guardians below but learned that the Key could reprogram them. He would activate Vector Sigma and give life to the Stunticons.
When he returned to Earth, Shockwave informed him that the Key possessed a different power on Earth which was capable of destroying it. Megatron accidentally learned, while studying the Key, that it was capable of transforming matter into metal. He began the process of transforming Earth into another Cybertron but was defeated by the Autobots. Silverbolt would later destroy the Key after he forced Megatron to drop it which reversed its effect on Earth.
Beast Machines cartoon
Using the Diagnostic Drone, Tankor was able to obtain the code that formed the basis for the Key and materialize copies of it. He shot Nightscream with it, which caused the Maximal to turn to technomatter and go insane. The condition was contagious, as anyone Nightscream touched also became technomatter. Pleased with the results, Tankor faked his own death, knowing that Megatron would find the decoy corpse and plunder the Key program from it. Tankor hoped that Megatron would use it to destroy all organic and techno-organic life on Cybertron—including Megatron himself, who still possessed his dragon beast mode.
Eventually, after Tankor gave Optimus Primal the key to activate the Plasma Energy Chamber in an attempt to force Megatron's hand, both Megatron and Primal activated their doomsday weapons, apparently disintegrating all three of them, but (thanks to a last-second change of mind from Primal) leaving Cybertron intact.
Primal would be resurrected by the Oracle shortly afterward, while Megatron would survive in a purely organic form. Rhinox, however, was lost to the Allspark.
Prime cartoonOptimus Prime entrusted Jack with the key as a sign that he wasn't coming back, as his normal self. [Episode needed]
After Optimus released the energy within the Matrix of Leadership to defeat Unicron, he reverted back to his pre-prime persona of Orion Pax. This was due to the fact that the Matrix possessed the collective knowledge of all the previous Primes and by releasing the energy, he lost all that knowledge, including his own. In order to return Optimus's memories, Jack traveled to Cybertron to locate the Vector Sigma Chamber.
Once the chamber was found Jack "downloaded" the collective wisdom of the Primes into the Key and returned to Earth where he "uploaded" it back into the Matrix. This granted Optimus access to all the information, including his lost memories.
- Alpha Trion (Timelines, 2007)