The Guardians of the Realm were a mighty protective force, an order of heroes, which was founded after the defeat of a demonic invasion. About 1200 years ago, the order was seemingly annihilated in a final decisive battle against evil.

The Guardians set great value upon preserving all knowledge of their time, so future generations would know how to deal with another invasion. For this purpose, they built libraries all over the world to house the records. Unfortunately, their downfall took place before they could finish their work.

SPOILER! (select text to reveal the spoiler)
Actually, the Guardians themselves weren't killed; instead they entered a state of hibernation, ready to be resurrected when the time was right. After the abyssal forces had turned against themselves and retreated, followed by hundreds of peaceful years, this was the case when Karzul entered the Material Realm, planning to enslave humanity.