A very interesting site:
Naqsh-e Rostam, the site where the inscription was found, was a necropolis and religious center about 12 km northwest of Persepolis, the ceremonial capital of the Achaemenid Empire (550-300 BC). However, some of the relics on the cliffs may date as far back as the Pre-Iranian Elamite Period (2700-539 BC).And, of course, there are also remains from the Sassanian (Sasanian) era, on which time see here and here.
Four huge tombs from the Achaemenid period, including the tomb of King Darius the Great (550-486 CE) hewn into the cliff face and a square-shaped building made with stone blocks from the Sassanian which may also have had a significance in burial rites are among the most important relics of the necropolis.
The inscription is in Pahlavi (Middle Persian), on which more here and here. The article discusses the challenges of deciphering inscriptions in this language. Alas, there is no indication of what the inscription says.
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