Each of the expressed streams of magic has an associated magical plane. Mages with exceptional levels of skill and power can learn to open gateways onto the planes. Such power is extremely rare. Only a handful of mages in a generation will have this ability.
Every plane has a guardian. It is thought that the guardians were once planewalkers. Existing in a purely magical environment changed their nature to such an extent that they can no longer leave their plane, but within it they have virtually unlimited power.
A planewalker can use the powers of their plane to travel vast distances in minutes. With permission from the guardian they can take others on the same journey. This can be dangerous. The plane of Death is not somewhere a mortal should linger for long without protection.
The plane of Death is unique in that it is split into two, the outer and inner planes. The outer plane is a staging point for the spirits of the dead before they are ushered on to their final rest. Some spirits become trapped at this point and must be guided onto the inner plane by Death mages.