Immortality shows itself in several forms.
The latest generations of manifold denizens have achieved a limited form of immortality. A denizen no longer dies of old age and can naturally or magically regenerate the most grievous of wounds, many that other species would consider fatal. However, a few very dangerous fatal diseases among the denizens still exist, and a denizen can be killed by completely removing its head or by crushing its entire body.
When a particularly insightful mortal of any imperfect species dies, it is given the chance to go to Hades to continue investigating a possible higher existence. If it chooses so, it then takes the form of a vampire and continues its existence there. A vampire is immortal in the sense that it can no longer die due to old age, hunger, or disease. Because it is already dead, anything short of physical destruction that would normally kill a living creature has negligible effect on it. It is thus now animated nonliving matter, or "undead." However, having already died, it is devoid of life energy and may leave Hades only at the risk of going mad with hunger for the life energies of others.
No one has achieved perfect immortality except the denizens, and in their case, in an ironic twist, only imperfect (i.e. non-denizen) beings may be made perfectly immortal. This is accomplished by turning the being into a Chart between worlds. Because a Chart exists between only two worlds, if a denizen is made into a Chart, it can no longer utilize other realities to define its existence, as it is bound to only two. Therefore, any denizen made into a living Chart is made imperfect and loses its near-omnipotent abilities.
A very select few have the theory that a denizen may be made imperfect, turned into a living Atlas, and then made a denizen again, since an Atlas is bound to all realities. However no denizen is bold enough to volunteer.