From the epics · 4 tellings · Pan-India, The Himalaya
Nahusha on Indra's throne
While Indra hides in a lotus stalk, the human king Nahusha rules heaven, demands Shachi, has sages carry his litter and is cursed to fall as a serpent.
Shalya tells Yudhishthira. Nahusha, crowned for his virtue and given a strength-draining gaze, grows proud and summons Shachi. Brihaspati shelters her; she gains time, calls up Divination and finds Indra in a lotus stalk on an island lake north of the Himalaya. Indra's plan is that Nahusha should come to her in a litter borne by sages. Agni, sent by Brihaspati, finds Indra in the waters; the gods' praise makes him grow again; he promises shares to Kubera, Yama, Varuna and Agni. Agastya arrives to say Nahusha called the hymns false, set a foot on his head and was cursed to be a serpent for ten thousand years.
Mahabharata, Vana Parva 178-181 (the serpent and Yudhishthira)
The HimalayaAll traditions
A huge serpent in a Himalayan cave seizes Bhima. Yudhishthira finds them; the serpent says he is Nahusha, son of Ayu and their ancestor, and will free Bhima if his questions are answered. Yudhishthira says a Brahmin is known by truth, charity, forgiveness and conduct, not birth. Nahusha then tells his own fall: prosperity intoxicated him, a thousand sages drew his chariot, his foot touched Agastya, who cursed him; as he fell he begged pardon and was told Yudhishthira would release him. He lets Bhima go, sheds the serpent body and returns to heaven.
What is different here:
Told by Nahusha himself; no Shachi, no lotus stalk, no stratagem; the curse ends on meeting Yudhishthira, not after ten thousand years; the lesson is that conduct, not birth, makes a Brahmin.
Mahabharata, Anushasana Parva 99-100 (Bhrigu in Agastya's hair)
Pan-IndiaAll traditions
Bhishma tells it to explain offerings of lamps and incense. Nahusha at first keeps every rite in heaven, then stops out of pride and loses energy. Brahma's boon makes anyone he sees powerless, so no sage can curse him. Bhrigu hides in Agastya's matted locks, unseen. Nahusha yokes Agastya on the bank of the Sarasvati, goads him and kicks his head with the left foot; Bhrigu, whom Nahusha never saw, curses him to fall as a snake.
What is different here:
Bhrigu, not Agastya, speaks the curse, and a reason is given why Agastya could not; no Shachi; the frame is ritual duty.
After Vritra's death Indra flees the personified sin to Lake Manasa and lives a thousand years in the fibres of a lotus stem; Agni cannot bring his share through the water. Nahusha, ruling meanwhile, is blinded by power, makes advances to Indra's wife, is cursed by a Brahmin and becomes a snake. The Brahmins call Indra back and purify him with a horse sacrifice.
What is different here:
Three verses only; the lake is named Manasa and the hiding lasts a thousand years; the curser is unnamed.