How did Garshasp defeat Bahu?

How did Garshasp defeat Bahu?

A significant part of Asad Tusi's epic "Garshaspname" was added to the manuscript of "Shahname" by Abulqasim Firdavsi, stored in the National Library of Russia.

According to Asadi Tusi’s poem, Garshasp was a descendant of Jamshid, the son of Itrit and the daughter of Kurang, king of Zabulistan. He was born in the reign of Zahhak in Iran, who took the throne from Jamshid. Zahhak sent the young knight to India, where the king, submissive to Zahhak, was overthrown by a certain Bahu. Garshasp defeated Bahu in battle, and he sent his black servant, distinguished by his extraordinary strength and speed, on the pretext of delivering a letter to get closer to Garshasp and strike him with a poisoned dagger. However, Garshasp captivated the black man and together with him went secretly to the camp of Bahu and kidnapped him. Then the two of them brought the captive to the Indian king during the feast – this is the moment depicted in the miniature.

You can learn more about the topic in the book-album "Arts of the book in the 15th–17th-century Mawarannahr" (Volume XVII) in the series "Cultural Legacy of Uzbekistan in the World Collections". 

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How did Garshasp defeat Bahu?