Core Energy is the most common type of energy used for skills. With the exception of Sophie Foster, this is the only source of power non-ancients can master. This is first mentioned in Book 1: Keeper of the Lost Cities, where it is described to be the reason behind the power and strength of skills. When drawn upon, the elf using it feels a pull in their gut. It doesn't mix with mental energy, causing the forces to rebound when they come in contact. Such happened in the Ultimate Splotching Championship.