The Nether is our ID made manifest, a world just a step outside of ours. From which humans draw upon inspiration, it is a realm of incredible beauty and incredible terror. Inside the nether resides the morti - beings of manifest terror, anger, fear. Generally these morti are aimless, reacting to the realm, drawn to those mortal souls who wander too close to their sphere of influence.
When a newly awakened breacher enters the Nether for the first time they must face their darker half. The half born from nightmare, darkness and fear. Those that overcome this darker half become Guardians, those that embrace it are known as Warlocks.
Those that do neither never awaken. The survivors learn how to harness their essence - combination of their life and mental energies - and how to manipulate the Nether with it.