Petitioners are the spirits of the dead, brought by various means to dwell upon the Outer Planes. The spiritual essence of the living, each petitioner follows their own path, hoping to stand for the beliefs they held most prominently in life so that they may either merge with the essence of their new planar home, or achieve their own brand of greatness through ascension to a higher form..
When a mortal dies anywhere in the multiverse, the essence of their consciousness instantly leaves their body, heading for the Astral Plane; even those mortals that die in the Inner Planes have their spirit take this route through yet-unexplained means. This essence passes through an astral conduit, where according to current best theories, its mortal memories are stripped away, forming a memory core floating in the Astral until it eventually fades from that realm — some claim it merely disassociates, others claim it passes beyond the multiverse entirely. Once on the Outer Planes, the soul is directed by its beliefs towards the destination most befitting it, where it manifests itself in a form most often identical to the form they most preferred in life (though this varies from plane to plane and power to power). A soul's destination is most often the plane most befitting the individual's outlook in life, or the realm of the power that individual was most faithful to. Followers of some pantheons, however - the Celestial Bureaucracy and the Ennead being two major examples - enter a stage of judgement to determine the final destination for the individual. It should be noted that those that, in life, held a strong, fervent belief in the nonexistence of an afterlife for the most part leave behind nothing but a memory core, their beliefs carrying the remainder of their essence to the oblivion they expect. This differs in some spheres, where the strongly atheistic have other specific fates determined by the powers of that sphere.
Once formed, most petitioners have the same personalities they had in life, though nearly all memories of that life are lost. Some may have flashes of names or common mannerisms carried over, but for the most part a petitioner is entirely devoid of their living nature. As a petitioner continues along their path, however, the innate wisdom of the plane or realm's nature, or that bestowed by their power, tends to change them, bringing them more in line with the nature of their home. They become more and more focused on that nature, leading to the common stereotype of petitioners as single-minded and boring. However, as petitioners come from mortals, there is as much variation in their nature as there is amongst the living.
On most planes, few petitioners have much power, but a few carry abilities from life; even fewer manage to grow in their abilities after death. For the most part, though, petitioners put focus on spiritual growth over martial or magical, and so most don't bother pursuing such tasks. In addition, few wish to risk facing "death" in such a manner. Petitioners that are slain on their home plane are dispersed, their spiritual essence only partially merging. Those that die away from their home plane suffer an even worse fate, their essence simply dispersing into nothing. Of course, there are exceptions, especially depending on the nature of the plane and of the core belief the petitioners seek to embrace. Those that seek a warlike, combative, defensive, or protective nature will often pick up physical skills. Those that followed a god of the arcane will usually gain in magical ability, even gaining sorcerous talents that they lacked in life. While there is of yet no way to determine this for certain, those that believe that memory cores ascend rather than disassociate claim further that they tend to vanish at the moment the soul of their associated former being merges or disperses.
Of course, if a mortal is resurrected in some manner, this also disperses the petitioner form in a sense, pulling it back through the Astral alongside its memory core, losing any memories of its time in the planes in the process. But the planes and the powers don't like losing their petitioners, especially given that once alive, they may choose a different path. Thus, resurrection of any sort tends to be a difficult task, requiring great power (and, of course, consent of the raised). And those that have moved on from being a petitioner, either through merging or through ascension, can no longer be brought back by any means.
- On Hallowed Ground, pp.28-33