Kurma
Kurma,the tortoise,appeared in the Satyuga and it was the second avatar(incarnation) of Lord Vishnu.Indra the king of all gods once insulted a powerful sage called Durvasa by not accepting his gift(a garland),the sage got angry and cursed all the gods saying that they will lose all their powers and become powerless.
After the curse all the gods went to Lord Vishnu,Vishnu suggested that they need to drink nectar of immortality called as Sanjeevni so that all the powers of the gods come back and they become powerful again.For that they had to do sagar manthan(churning of ocean) which needed asuras help as well.When all the devas and asuras were churning the ocean using mount Mandara,the mountain started slipping in the ocean.hence Lord Vishnu took the form of Tortoise so that the mountain does not slip in the ocean and provided it a support by bearing the mountain on his back.In that way all the gods got back their lost powers.
