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.
In Predacons Rising, Unicron makes a return in Megatron's new improved body. He revives Cybertron's dead Predacons and heads for Cybertron's core to destroy Primus so that he can begin a new age of chaos. The Autobots manage to hold him off for a little while, with the aid of Predacon allies Predaking, Skylynx and Darksteel. Though their attempt failed as their Predacon ancestors took them to the core as they dived into the Well of Allsparks. Optimus Prime returns and holds off Unicron. Optimus tried to get the Allspark from Unicron, but failed as Unicron managed to get it from him. As Unicron opened the vessel, there was nothing there, but a trap. Unicron's antispark was locked into the vessel trapping him for good. Later Optimus told his fellow Autobots that he must finish his quest to restore Cybetron with the Matrix of Leadership. As Optimus says his last goodbyes, he flew directly into the core opening his chest to reveal the Matrix of Leadership. Suddenly sparks began to fly from the well. The Autobots were amazed as they saw the sparks, including Optimus' spark.