In some Middle Persian texts, there is a story about Gayōmart which is not mentioned in older texts such as the Avestan texts, and also it is difficult to find evidence of it in Persian and Arabic epics. This story refers to the killing of a demon named Arzūr by Gayōmart, the work which is counted among the most important works of Gayōmart on the day of Khordād of the month of Farvardīn (the sixth day of month Farvardīn). In addition, in Avesta, Arzūr is the name of a mountain where the demons gather and in Pahlavi texts such as Bundahišn, Šāyist Nē-šāyist and Pahlavi Rivāyat, it has been mentioned as the gate of hell and the passage of Ahreman and demons. In this article, an attempt is made to explain the relationship between mount Arzūr, the door of hell and the demon Arzūr, by searching for the name Arzūr in Avestan and Middle Persian texts, and also in the Šāh-nāma and Ātār al-Bāqīah. As far as the author is concerned, Šāh-nāma story can be reconstructed with respect to Pahlavi sources and Bīrūnī’s account like this: Gayōmart killed Kharzūr/Arzūr’s demon; After that, Ahreman sent Kharzūrān (= son of Kharzūr) to fight with Gayōmart and in this battle, Siamak was killed. Finally, Siāmak’s son took revenge on Kharzūrān and killed him.