Most people only know of occasions in which two different incarnations of Vishnu met, for instance the encounter of Rama and Parashurama as described in the Bala Kanda of the Ramayana. What is not as well known, however, is that Vishnu's incarnation Krishna once met Vishnu himself! The story is described in both the Harivamsa and the Srimad Bhagavatam.
Once when Arjuna was visiting Dwaraka, he saw a Brahmana complain that all his sons were dying as soon as they were born. So Arjuna offered to stand guard and prevent any more of his children from being taken away by Yama god of death. But when the next child was born, it magically floated up into the sky. Arjuna was baffled by this, and he asked for Krishna's help in retrieving the Brahmana's sons from the afterlife. Krishna obliged and took Arjuna in his chariot. They left the realms of Man until they came to a region of darkness. Krishna cut through the darkness with the illumination of his Sudarshana Chakra, and soon they entered the brilliance of Brahman, after which they finally encountered Vishnu himself. Vishnu explained that he was the one who took away the Brahmin's sons, just to get Krishna and Arjuna to come there, as they were the next births of his incarnation Narayana and his twin brother Nara (whom I discuss here and here). Finally Krishna and Arjuna returned to Dwaraka with the Brahmana's sons restored to life.
But my question isn't about the story of how they met Vishnu, but rather about how Vishnu looked. Here is how the Srimad Bhagavatam describes him:
Arjuna then saw the omnipresent and omnipotent Supreme Personality of Godhead, Mahā-Viṣṇu, sitting at ease on the serpent bed. His bluish complexion was the color of a dense rain cloud, He wore a beautiful yellow garment, His face looked charming, His broad eyes were most attractive, and He had eight long, handsome arms. His profuse locks of hair were bathed on all sides in the brilliance reflected from the clusters of precious jewels decorating His crown and earrings. He wore the Kaustubha gem, the mark of Śrīvatsa and a garland of forest flowers. Serving that topmost of all Lords were His personal attendants, headed by Sunanda and Nanda; His cakra and other weapons in their personified forms; His consort potencies Puṣṭi, Śrī, Kīrti and Ajā; and all His various mystic powers.
My question is, why is Vishnu described as having eight arms? He is usually described in Hindu scripture as having four arms, not eight.
Is this one of Vishnu's manifestations, like the famous Vyuha forms of Vishnu discussed in Pancharatra texts? Are there any other scriptures that describe an eight-armed form of Vishnu?
The Supreme Lord, who sits on the back of the bird Garuḍa, touching him with His lotus feet, holds eight weapons — the conchshell, disc, shield, sword, club, arrows, bow and ropes. May that Supreme Personality of Godhead protect me at all times with His eight arms. He is all-powerful because He fully possesses the eight mystic powers.