The Core is the heart of Cybertron located in the center of the planet and is considered as sacred grounds by the Cybertronians as it produced vast quantities of Energon, essential to the continued existence of the Transformer race and Cybertron. The Core is the heart of Primus himself.
It resembles a massive sphere construct, the Core possess its own consciousness and mind allowing to speak as well as act. It is Primus himself.
While not much is known of Primus's origins, he created the Thirteen before fading away into hyper sleep to turn the tide of his war against Unicron.
Primus is generally accepted to exist though his 13 children are considered a myth by some. Throughout Cybertron devices carry out his will: the AllSpark creates the protoforms in the Well of All Sparks. The Sonic Canyons are believed to be his ears, feeding him knowledge of the universe via Vector Sigma, though that is actually beneath the gladitorial arenas of Kaon. And the Core produces his own physical emanation, Energon.
During the civil war, the AllSpark was ejected into space to keep it out of Decepticon hands, eventually floating into the Milky Way, while the Core was corrupted by the Dark Energon-imbued Plasma energy Chamber. 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. When the Autobots left Cybertron, the last original Prime Alpha Trion remained, stating Vector Sigma would keep him alive.
According to legend, at the beginning of time, Unicron and Primus battled each other for eons, with neither side remaining victorious for long. Until, that is, Primus created the Thirteen: the original Primes. These Thirteen defeated Unicron and cast him into space. Primus then became one with the Core of Cybertron, and brought life to new Cybertronians through the Well of All Sparks. Optimus Prime would later speak of Unicron as being a physical being that had likely transcended physical form, with Primus presumably being the same. When Primus was poisoned from the result of the war, Orion Pax came into contact with Primus and was given the Matrix of Leadership. It was revealed that during the war, Optimus removed the AllSpark, the source of new life creation on Cybertron, and sent it into deep space.
Many thousands of years later, Primus was restored by Team Prime when they used the Omega Lock built into the Nemesis to restore Cybertron. However Primus's awakening would cause his slumbering arch foe Unicron to reawaken and take control of Megatron's body, flying to Cybertron to destroy him. Raising an army of Terrorcons, Unicron sent them into the Well of All Sparks to destroy Primus, but they themselves disintegrated, within inches of the Core, when Unicron was defeated