The Silent Core is a huge orb of pure dark and negative energy from the very center of Vestroia that initially trapped Naga within it. Naga slowly evolved until he took control and possessed the Silent Core. He used a portal to get there. When Naga fully evolved, his power-level went up to 1,000 Gs.
He became undefeatable until Infinity Dragonoid and the other main Bakugan used the Dextra Attack Ability Card to ultimately defeat Naga. Drago revealed that when the Core split and Vestroia became the world of the six attributes, the enormously strong dark energy created sent powerful Bakugan to the dimension of Neathia and Gundalia.
In the 2009 Video Game, Hal-G extracted a portion from the Silent Core's power and it was called the Silent Orb. It was going to be Vladitor's reward for destroying Leonidas. When Vladitor failed, Hal-G told him he would not receive the orb. Vladitor took the orb from him anyway and evolved into Battle Ax Vladitor. The Silent Orb's powers were exhausted during Vladitor's battle with Omega Leonidas and his remaining strength was used to save Leonidas from being destroyed.