Thousands of years ago, the Gods were locked in a divine war as the forces of good, evil and chaos clashed. After millennia of conflict, a truce was called.
Towards the end of the war, the Gods saw that foot soldiers could be used to turn the tide, so they bred and created mortals to serve in their armies. However, as mortality grew in number, the power of the Gods began to fade.
Once the Gods realized that the combined power of the mortals they created was greater than their own, they split the planes and created three separate realms, each protected by barriers to prevent the mortals from uniting under one banner.
These realms were Reliquiae, the Feywild, and the Shadowfell.