The concept of the petitioner first appeared in second edition for the Planescape setting in the original Planescape Campaign Setting boxed set (1994), and appeared throughout the Planescape line of products.
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. Most petitioners, however, are content to simply live out their eternity in whatever form their god has chosen for them.
In Planescape a petitioner loses all knowledge of their previous life. They are also unable to advance beyond the zeroth or first level of experience.
In Dungeons & Dragons 3rd edition (also referred to as 3.0) and 3.5 edition, Petitioner is a template from the Deities and Demigods book. The template uses the base stats of a creature and remakes it so that it has 2 HD and nothing more. A deity can have an unlimited number of such templates servants.