The Letter of Tansar (Persian: نامه تنسر) was a 6th-century Sassanid propaganda instrument that portrayed the preceding Arsacid period as morally corrupt and heretical (to Zoroastrianism), and presented the first Sassanid dynast Ardashir I as having "restored" the faith to a "firm foundation."
"[1] The document seems to have been based on a genuine 3rd-century letter written by Tansar, the Zoroastrian high priest under Ardashir I, to a certain Gushnasp of Parishwar/Tabaristan, one of vassal kings of the Arsacid Ardavan IV.
[2] Representative of those charges is the accusation that Ardashir "had taken away fires from the fire-temples, extinguished them and blotted them out."
The legend that the Arsacid Parthians had allowed Zoroastrianism to fall into neglect stems from the same period.
The importance of the Letter of Tansar was first perceived by James Darmesteter, who published the first critical translation of it in 1894.