jyotiḥ sva-dhāmnā jvalayad diśo daśa
pratīkṣya khe ’vasthitam īśa-nirgamaṁ
viveśa tasmin miṣatāṁ divaukasām
Apparently the serpent named Aghāsura, because of having received association with Kṛṣṇa, attained mukti by entering Kṛṣṇa’s body. Entering the body of Kṛṣṇa is called sāyujya-mukti, but later verses prove that Aghāsura, like Dantavakra and others, received sārūpya-mukti. This has been broadly described by Śrīla Viśvanātha Cakravartī Ṭhākura with references from the Vaiṣṇava-toṣaṇī of Śrīla Jīva Gosvāmī. Aghāsura attained sārūpya-mukti, being promoted to the Vaikuṇṭha planets to live with the same four-armed bodily features as Viṣṇu. The explanation of how this is so may be summarized as follows.
The effulgence came out from the python’s body and became purified, attaining spiritual śuddha-sattva, freedom from material contamination, because Kṛṣṇa had stayed within the serpent’s body, even after the serpent’s death. One may doubt that such a demon, full of mischievous activities, could attain the liberation of sārūpya or sāyujya, and one may be astonished about this. But Kṛṣṇa is so kind that in order to drive away such doubts, He had the effulgence, the individual life of the python, wait for some time in its individuality, in the presence of all the demigods.
Kṛṣṇa is the full effulgence, and every living being is part and parcel of that effulgence. As proved here, the effulgence in every living being is individual. For some time, the effulgence remained outside the demon’s body, individually, and did not mix with the whole effulgence, the brahmajyoti. The Brahman effulgence is not visible to material eyes, but to prove that every living being is individual, Kṛṣṇa had this individual effulgence stay outside the demon’s body for some time, for everyone to see. Then Kṛṣṇa proved that anyone killed by Him attains liberation, whether sāyujya, sārūpya, sāmīpya or whatever.
But the liberation of those who are on the transcendental platform of love and affection is vimukti, special liberation. Thus the serpent first entered the body of Kṛṣṇa personally and mixed with the Brahman effulgence. This merging is called sāyujya-mukti. But from later verses we find that Aghāsura attained sārūpya-mukti. Text 38 explains that Aghāsura attained a body exactly like that of Viṣṇu, and the verse after that also clearly states that he attained a completely spiritual body like that of Nārāyaṇa. Therefore in two or three places the Bhāgavatam has confirmed that Aghāsura attained sārūpya-mukti. One may then argue, How is it that he mixed with the Brahman effulgence? The answer is that as Jaya and Vijaya, after three births, again attained sārūpya-mukti and association with the Lord, Aghāsura received a similar liberation.