Testament Tatar and the Muslim of the Grand Duchy of Lithuania, Bohdan Muchla, was written in Polish in 1770, but written in Arabic. It is an interesting example of a secular, administrative and legal document which uses the hermetic Arabic alphabet, usually used by the Lithuanian-Polish Tatarów to record religious texts. The article is written by two researchers in the literature of Muslims of the Grand Duchy of Lithuania: the historian — heraldic and genealogist — and the philosophy — slavist and citalist. Their scientific cooperation in reading and interpreting the historical and philosophy of the unique text resulted in interesting conclusions such as the genealogy of the Tatar Rode of Muchlów and the procedure for the so-called nobility. Deliberate changes to the content were also revealed when reading and ‘translating’ the text of the will for the purposes of the Committee of Inferences. The article contains a photocopy of the will and its transliteration into Latin characters in two versions, strict and spelled out.