Priests are NPCs found near certain statues in towns, cities, and the wild. Currently they all look like Khurites dressed in some fur. While you are dead, you can walk up to a Priest and hit "R" while facing them to interact. This will revive you from the etherworld.
You can also use "Beta Suicide" once you are dead to find a priest. WARNING: Beta suicide does NOT find the nearest priest (it is bugged). For example, suiciding in fabernum might get you to meduli.