The sword obtained by crowned prince Arslan, it appears in ⇒Arslān Senki ["The Arslan War-Chronicles"] series written by Japanese author Yoshiki Tanaka (b. 1952), a work inspired by the medieval Persian epic Shah Nameh by Ferdausi, from which many of the proper names for the characters and geographic settings are taken.

In the fantasy novel, Ruknābād is the name of the sword interred with Kai Khosrau to seal the serpent king Zahhak in Mt. Damavand. It is obtained by Prince Arslan to gain legitimacy to his succession.

§ Plot-line:

warning: contains spoiler if you haven't seen or read the work. I'm also not complying at all with the spellings used in "The Heroic Legend of Arslan" animated series (in English).

Years ago, the serpent-king Zahhak reigned over a millenium of cruelty and horror. Out of his shoulders grew a pair of serpents which demanded to be nourished with sacrificial human brains. Then the hero-king Kai Khosrau appeared, capturing the serpent-king on the plains of Mazandarān . Zahhak could not be destroyed, but "he was stuffed in a cave deep in the earth, bound with thick chains, the tendons of his hands and legs severed, with twenty thick slabs of rock piled on top". And Khosrau's treasure-sword Ruknābād was thrust into the earth to seal the evil king.
When Kai Khosrau died at forty-five years of age, he left a will that he be buried in full armor, so that the sword was exhumed from Mt. Damavand and reburied inside his coffin. (Arslān Senki Book 2, Ch. 4, VII.)

Hirmes' attempt to obtain:
Prince Hirmes (Lord Silver Visor), a rival claimant to the Parsian throne, opts to legitimize his claim to the throne by going to Mt. Damavand and obtain the sword. However when the sword is dug up the mountain rumbles and the winds swirl with hell-fury. Gīv the minstrel who had been on a scounting mission for Arslān's sake witnesses this, and reproaches the prince, saying he has been deemed an unworthy recipient of the sword, and must replace it straighway or, as every toddler in Pars knows, the serpent king will arise again. Hirmes temporarily hands Ruknābād over to Zand, one of his lieutenants, but Zand is forced to agree with the annoying minstrel on this matter, and casts the sword deep into the chasm. (A. S. Book 5, Ch. 2, II-)

Arslān's momentous decision:
The fourteen-year old Crowned Prince Arslān, having assumed acting rulership while the king was held prisoner, has seen a spurt of growth. He has been enlightened about the inequities and inadequacies of the present system of governance. He also sees how King Andragorus, though an able war-wager, has brought needless suffering on account of his short-fused nature and tyrannical tendencies.
Then the revelation is made that Arslān is not the trueborn son of the king, thus not of the royal line of descent at all (even though one cannot rule out the possibility of a drop or two of royal blood being mingled in his heritage by happenstance, as the sorcerer of the serpent cult points out).
Though lacking the legitimate claim by consanguinity, the prince comes to a momentous decison. He shall go to Mt. Demavand to obtain the sword Ruknābād which will be the emblem of chosen heir. Visiting Kai Khusrau's burial ground, he stands whipped by the furious gale and rain, and beckons to the spirit of the hero-king. Amid a burst of light, the sword appears in his hands. His fellow travelers kneel, and hail the shah. The spirit of the hero-king has deemed him worthy successor to the kingdom. (A. S. Book 7, Ch. 5)