The Heartless are beings of darkness that manifest in two forms, "Pureblood" and "Emblem". While most Heartless are, in fact, manifested hearts, they behave entirely devoid of emotion and thus were named "Heartless" Trust me when I tell you these beings are no joke. If you ask them to hurt someone, they will do so, without mercy, no heart means no feelings. their very nature, as manifestations of the destructive force of darkness, makes them naturally suited for conquering and destroying anyone that stands in their way. Their natural ability to travel to anywhere they choose through the use of Corridors of Darkness makes it virtually impossible to fully defend against them, and the method in which they travel and the power they use makes it practically impossible to cut them off at the source. Even fighting them once they appear is made difficult due to their immunity to standard spells.