A mortal who has died and reformed on the plane of his alignment and/or deity without memory of his former life. A petitioner's ultimate goal is to become one with the plane he's occupying, although no one (not even the petitioner) knows the whole dark of this.
In the Dungeons & Dragons fantasy role-playing game, a petitioner is the soul of a deceased mortal. Upon its death, a mortal creature that served a deity faithfully will reform on the home plane of that deity. If the new petitioner continues to serve its deity, it can be made even more powerful.