[excerpt from Encyclopaedia Theologica: A Panoptic Primer on the Pantheon, Planes, and Patrons of Ylan, 5th ed., by the gnomish scholar Miriam Busslewickett]
“Transcendence” can refer to the Transcendent plane itself, the dwelling place of deities and Patrons. Its demi-planes include Heaven, Purgatory, Hell, and Patron-specific planes.
The term can also refer to the process by which certain mortals have ascended beyond the Prime Material plane to become Patrons. Little is known of what forces are at play in raising a mortal to Transcendence, but it is known to have occurred only a handful of times in recorded history. Otherwise, mortals who end their lives on the Prime Material plane return to the plane of Oblivion.
Movement between the Prime Material and Transcendent planes is almost always one way; only through coordinated, simultaneous efforts on both planes can a Transcendent entity cross back to the Prime.
The Astral plane is common to the Prime and Transcendent planes and is the primary means of communication between mortals and their Patrons.