Kṛṣṇa is known as Yogeśvara, as stated in the last portion of the Bhagavad-gītā. Yogeśvaro hariḥ: all mystic powers are under His control. In our experience we can see many human beings who have yogic mystic power and who sometimes perform very wonderful acts, but Kṛṣṇa is understood to be the master of all mystic power. Therefore, when He saw that His horses were hesitant to proceed into the darkness, He immediately released His disc, known as the Sudarśana cakra, which illuminated the sky a thousand times brighter than sunlight. The darkness of the covering of the universe is also a creation of Kṛṣṇa's, and the Sudarśana cakra is Kṛṣṇa's constant companion. Thus He penetrated the darkness by keeping the Sudarśana cakra before Him. Śrīmad-Bhāgavatam states that the Sudarśana cakra penetrated the darkness just as an arrow released from the Śārṅga bow of Lord Rāmacandra penetrated the army of Rāvaṇa. Su means "very nice," and darśana means "observation"; by the grace of Lord Kṛṣṇa's disc, Sudarśana, everything can be seen very nicely, and nothing can remain in darkness. Thus Lord Kṛṣṇa and Arjuna crossed over the great region of darkness covering the material universes.
Arjuna then saw the effulgence of light known as the brahma-jyotir. The brahma-jyotir is situated outside the covering of the material universes, and because it cannot be seen with our present eyes, this brahma-jyotir is sometimes called avyakta. This spiritual effulgence is the ultimate destination of the impersonalists known as Vedāntists. The brahma-jyotir is also described as ananta-pāram, unlimited and unfathomed. When Lord Kṛṣṇa and Arjuna reached this region of the brahma-jyotir, Arjuna could not tolerate the glaring effulgence, and he closed his eyes. Lord Kṛṣṇa's and Arjuna's reaching the brahma-jyotir region is described in the Hari-vaṁśa. In that portion of the Vedic literature, Kṛṣṇa informs Arjuna, "My dear Arjuna, the glaring effulgence, the transcendental light you are seeing, is My bodily rays. O chief of the descendants of Bharata, this brahma-jyotir is I Myself." As the sun disc and the sunshine cannot be separated, Kṛṣṇa and His bodily rays, the brahma-jyotir, cannot be separated. Thus Kṛṣṇa claims that the brahma-jyotir is He Himself. This is clearly stated in the Hari-vaṁśa, when Kṛṣṇa says ahaṁ saḥ. The brahma-jyotir is a combination of the minute particles known as spiritual sparks, or the living entities, known as cit-kaṇa. The Vedic words so ’ham, or "I am the brahma-jyotir," can also be applied to the living entities, who can also claim to belong to the brahma-jyotir. In the Hari-vaṁśa, Kṛṣṇa further explains, "This brahma-jyotir is an expansion of My spiritual energy."