What is the justification given?

O Yaja, perform that sacrifice by means of which I may obtain a son invincible in battle and capable of slaying Drona. Ready am I to give thee ten thousand kine.' Hearing these words of Drupada, Yaja said, 'So be it.' Yaja then began to recollect the various ceremonies appertaining to the particular sacrifice. And knowing the affair to be a very grave one, he asked the assistance of Upayaja who coveted nothing. Then Yaja promised to perform the sacrifice for the destruction of Drona. Then the great ascetic Upayaja spoke unto king Drupada of everything required for the grand sacrifice (by aid of fire) from which the king was to obtain offspring. And he said, 'O king, a child shall be born unto thee, endued, as thou desirest, with great prowess, great energy, and great strength.'

"The Brahmana continued, 'Then king Drupada, impelled by the desire of obtaining a son who was to slay Drona, began, for the success of his wish, to make the necessary preparations. (And when everything was complete) Yaja, after having poured libations of clarified butter on the sacrificial fire, commanded Drupada's queen, saying, 'Come hither, O queen, O daughter-in-law of Prishata! A son and a daughter have arrived for thee!' (draupadis birth was a surprise to them all)Hearing this, the queen said, 'O Brahmana, my mouth is yet filled with saffron and other perfumed things. My body also beareth many sweet scents; I am hardly fit for accepting (the sanctified butter which is to give me offspring). Wait for me a little, O Yaja! Wait for that happy consummation.' Yaja, however, replied, 'O lady, whether thou comest or waitest, why should not the object of this sacrifice be accomplished when the oblation hath already been prepared by me and sanctified by Upayaja's invocations?'

"The Brahmana continued, 'Having said this, Yaja poured the sanctified libation on the fire, whereupon arose from those flames a child resembling a celestial who possessing the effulgence of fire, was terrible to behold. With a crown on this head and his body encased in excellent armour, sword in hand, and bearing a bow and arrows, he frequently sent forth loud roars. And immediately after his birth, he ascended an excellent chariot and went about in it for some time. Then the Panchalas in great joy shouted, 'Excellent, Excellent.' The very earth seemed at that time unable to bear the weight of the Panchalas mad with joy. Then, marvellous to say, the voice of some invisible spirit in the skies said, 'This prince hath been born for the destruction of Drona. He shall dispel all the fears of the Panchalas and spread their fame. He shall also remove the sorrow of the king.' And there arose, after this from the centre of the sacrificial platform, a daughter also, called Panchali, who, blest with great good fortune, was exceedingly handsome. Her eyes were black, and large as lotus-petals, her complexion was dark, and her locks were blue and curly. Her nails were beautifully convex, and bright as burnished copper; her eye-brows were fair, and bosom was deep. Indeed, she resembled the veritable daughter of a celestial born among men.

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[paragraph continues] Her body gave out fragrance like that of a blue lotus, perceivable from a distance of full two miles. Her beauty was such that she had no equal on earth. Like a celestial herself, she could be desired (in marriage) by a celestial, a Danava, or a: Yaksha. When this girl of fair hips was born an incorporeal voice said, 'This dark-complexioned girl will be the first of all women, and she will be the cause of the destruction of many Kshatriyas. This slender-waisted one will, in time, accomplish the purpose of the gods, and along with her many a danger will overtake the Kauravas.' On hearing these words, the Panchalas uttered a loud leonine roar, and the earth was unable to bear the weight of that joyous concourse. Then beholding the boy and the girl, the daughter-in-law of Prishata, desiring to have them, approached Yaja and said, 'Let not these know any one else except myself as their mother.' Yaja, desiring to do good unto the king said, 'So be it!' Then the Brahmanas (present there), their expectations fully gratified, bestowed names upon the new-born pair, 'Let this son of king Drupada, they said, be calledDhrishtadyumna, because of his excessive audacity and because of his being born like Dyumna with a natural mail and weapon.' And they also said, 'Because this daughter is so dark in complexion, she should be called Krishna (the dark).'