Key to Vector Sigma
The Key to Vector Sigma, as its name implies, was the means by which the supercomputer Vector Sigma at the core of Cybertron (Seibertron) could be accessed and activated. For millions of years, the Key was in the possession of Alpha Trion, one of the eldest Autobots (Cybertrons), who kept it hidden within his lab.
In 1985, Megatron, seeking the Key, broke into Trion's lab and stole it. Using the Key (which, as Megatron discovered, enabled him to command ancient Centurion Droids), Megatron activated Vector Sigma and had it give life to his Stunticons. Bringing the Key back to Earth, Megatron discovered it had an unusual ability there- it could transform organic material into metal and robotics. The Decepticon (Destron) leader nearly used the Key to change Earth into a second Cybertron, until the Key was destroyed by a well-aimed shot from the new Aerialbot leader, Silverbolt. Thus, the Key had come to an end.
However, the Key's program remained within Vector Sigma, and centuries later, it was extracted by the Diagnostic Drone for Tankorr. After Tankorr used a duplicate of himself to test it, the Key program was given by the Drone to Megatron, who proceeded to install it in all his Vehicons and use it against the Maximals led by Optimus Primal. Eventually, Megatron opted to use the combined power of all his Vehicons' Key programs to eradicate all organic life and insure a world with mechanical purity. When he used this, a desperate Optimus Primal released the energies of the Plasma Energy Chamber, and the two energies mingled. Their combined power begin annihilating everything on Cybertron, both mechanical and organic. Optimus Primal sacrificed himself to absorb the Key and Plasma energies, saving Cybertron.
The Key program remained unused until the final battle between Megatron, when an empowered Megatron tried to use it to transform Cybertron's organic core into technomatter. Primal stopped him, and threw both himself and Megatron to fall into the core, changing Cybertron into a technorganic world.