Demon Cores are an external phylactery containing the heart and soul of a @Demon Lord. They look like glowing spheres of matter and demonic energy containing which extend the life of their masters indefinitely. They also allow them to manipulate matter and eventually let them access all sorts of unnatural powers. However, if a Demon Core is destroyed, the @Demon Lord who owned it will be destroyed in turn.
Demon cores are similar to a Dungeon Core. They an be used to summon servants, cast spells to alter reality momentarily and empower the demon lord. A demon lord that is destroyed will come back to life at his demon core. But if the demon core is destroyed, the demon lord will be destroyed for good.
@Demon Lords of the @Demon Lord cannot be destroyed unless their demon core is destroyed.