Core

The Core is located at the centre of the planet Cybertron and considered sacred ground by the Cybertronians as it produced vast quantities of energon that is essential to the continued existence of the Transformer race. Resembling a massive spherical construct, the Core possesses its own consciousness and mind allowing it to speak as well as act.

Exodus
The dying Sentinel Prime ordered Optimus to go to the Core and remove its infection. Optimus was accompanied by Bumblebee and Jetfire and began descending the Underworld through the empty Well of All Sparks. Horrified by the infestation, Bumblebee tore out the corruption spikes on the Core, allowing Optimus to remove the poisonous Plasma Energy Chamber. A voice that only Optimus could hear then told him he was too late, but it imbued his body with the Matrix, truly making him a Prime. It told him to leave Cybertron, but to also return one day, when the Matrix and the Core would act in concert to restore the planet.

Prime cartoon
After the Thirteen defeated Unicron, Primus became one with Cybertron's core and created life through the Well of All Sparks.

Much later, the civil war on Cybertron poisoned the very Core itself. Orion Pax journeyed there in the hope of reversing the effects and made contact with the ailing Primus, who bestowed the Matrix of Leadership upon him.

After winning the Great War and taking control of the Decepticon warship, the Autobots traveled to Cybertron via space bridge and used the newly constructed Omega Lock to fire a beam of perfected synthetic energon down the Well of All Sparks, directly into the core. The Synth-En revitalized the core, repairing it to the extent that it could once again sustain life on Cybertron.

War for Cybertron

 * Voice actor: Isaac C. Singleton, Jr. (English)

In the closing stages of the Great War, the Decepticon leader Megatron learnt of the existence of a powerful corruptive energy source known as Dark Energon located at an orbital station. After acquiring the station and manufacturing it through the Geosynchronous Energon Bridge, Megatron decided to corrupt the Core of Cybertron itself in order to win his war against the Autobots. To accomplish this feat, he first needed the Omega Key and thus laid siege to Iacon itself and journeyed into the Vaults where he battled Zeta Prime for the relic. It was only after the battle did he learn of Omega Supreme being the key to the Core and battled the gigantic Autobot as well as defeating him. With Omega Supreme defeated, Megatron forced the Guardian to open the Omega Gate and took his Decepticons to the Core. There, he used Corruptors to infuse the Core with Dark Energon which began to manifest all over the planet Cybertron and threatening its survival.

With the defeat of Zeta Prime, the High Council of Elders promoted Optimus to the position of Prime in order to lead the Autobots. Seeing the continued corruption of Dark Energon spreading across Cybertron, Optimus Prime decided to journey deep into the planet and purge the infestation at its source. Taking Ironhide and Warpath with him, they learnt that Space slugs were fighting a losing battle in cleansing the Core of Dark Energon. Furthermore, the trio of Autobots had to fight against a garrison of Decepticons left behind by Megatron to ensure that the Core remained corrupted. Despite their resistance, Optimus Prime and his team managed to reach the centre of Cybertron and kill a corrupted space slug, thus freeing the Core. There, the Core spoke to Optimus Prime and revealed that the corruption of Dark Energon had been too extensive and in order to conserve itself - the Core had to shut down and purge itself in a process that would take millions of years where it would no longer be able to produce large quantities of Energon.

This meant that the planet Cybertron would be barren as a result and insufficient energy would be present for those that remained on the surface. Despite claiming that the line of Primes had failed in their mission, the Core decided to impart the Matrix of Leadership to Optimus Prime stating that so long as it functioned; its own Spark would continue. With that done, Optimus Prime returned to the surface and began the Great Exodus of Cybertron, leaving only a small number of them behind who vowed to continue to fight Megatron's tyrannical rule.