Immortal units stubbornly resist the Underworld and will reform at their home province if slain. This takes time, and it won't happen if the home province belongs to someone else when it's supposed to occur. The amount of time taken can be viewed by clicking on the trait.
How complete is the reformation? It's akin to the reformation a Pretender God goes through when being recalled. Injuries that caused the death, such as not having a head, are fixed. Debilitating afflictions such as Feeblemind, Battle Fright, and Disease are also shed. Other afflictions are less likely to be mended. An Immortal Pretender who is "slain" never loses Dominion strength or Magic paths when coming back via their Immortality, though they don't gain new magic levels either.
Dominion Immortality is a less common trait, but one typically found on the living instead of the normal trait; the normal trait is typically found on the Undead. This version only operates within friendly Dominion.
Certain things can't be returned from via immortality. Soul Slay, for example, splits the soul and the mind from the body; the immortal thus cannot reform as that body. Dying in another plane (such as the Astral Plane for the Arena, the Void, the Inferno, and Kokytos) makes it so the immortal can't reform in this world, since their soul cannot casually switch planes. An Immortal Pretender slain in one of these conditions must be Recalled the regular way, with the regular chances of weakening or insight.