Srimad Bhagvata Mahapurana: Book 6: Chapter 14: Verses 1-16
Pariksit submitted : How did (such an) unflinching devotion to the almighty Narayana appear in the heart of the sinful Vrtra, whose nature, O holy Brahmana, was predominated by Rajas and Tamas? Devotion to the feet of Lord Visnu (the Bestower of Liberation) does not ordinarily develop in the heart of (even) gods, whose intellect is (generally) pure, and sages, whose mind is untainted by sin. Living beings in this universe are as numerous (innumerable) as there are particles 0f dust. Of them, only a few human creature and other (higher) beings, as a matter of fact, practise virtue. Of them (again) seekers of liberation are ordinarily only few, O jewel among the Brahmanas ! And among thousands of those seeking release scarce one is completely rid of attachment (to his home etc.) and attains success (in the shape of Self-Realization). Even among tens of millions of those who have been rid of identification with the body etc., and even realized the Self, he whose mind is perfectly serene (entirely free from the craving for sense-gratification) and solely devoted to (the feet of) Lord Narayana is most difficult to find. How, then, did the sinful Vrtra, the tormentor of all the worlds, who gratified by his valour on the battle-field (even) Indra (the thousand-eyed lord of paradise), remain so steadfast in (his) devotion to the Lord (the Attractor of all) in the midst of a fierce combat ? Great is our doubt in this matter and so is our eagerness to hear about it, O Master. ( Suta continued : The glorious Suka (the son of Badarayana, more popularly known as Vedavyasa) welcomed the relevant enquiry of the devout king Pariksit, when he heard it, and then made the following reply.
Sri Suka said : Hear attentively, O king, the following legend as heard (by me) from the mouth of (my father,) the sage Vedavyasa (who was born in an island), as well as from the sages Narada and Devala. In the Surasena country (the tract lying about the city of Mathura), so the tradition goes, there was a king, called by the name of Citraketu, O Pariksit, who ruled over the entire globe and to whom the earth yielded everything sought after (by him). He had a crore wives; but, though capable of procreation, the emperor got no issue by (any of) them. Anxiety now laid hold of Citraketu, who, though (fully) endowed with beauty, generosity, youth, noble lineage, learning, (universal) sovereignty, imperial fortune and all other virtues, was (yet) the husband of barren ladies. All his riches, fair-eyed queens and (even the sovereignty of) this earth did not prove a source of delight to that ruler of the entire globe. Ranging through these worlds, however, the glorious sage Angira on one occasion came to his house by chance. Having honoured him with due ceremony by rising to greet him and offering him articles of worship etc., the emperor sat with a collected mind near the sage, who was now comfortably seated, having been (fully) entertained as a guest. The great sage (Angira). O emperor, showed every courtesy and consideration in return to Citraketu, who sat close to him on the (bare) ground, bent low with modesty, and, calling his attention (to him), spoke the following words.