Lord Vishnu is one of the great deities of the Hindu trinity, constituting as the preserver. His ten incarnations and the corresponding situations which led to such incarnations are as under:
(i) Matsya (fish): to save the sage Manu when the world was engulfed in a deluge.
(ii) Kurma (tortoise): to support the mountain Mandhara on his back so as to enable the gods to agitate the milk ocean and obtain ambrosia.
(iii) Varaha (boar): to destroy the demon Hiranyaksha with the powerful tusks and thereby save the earth from being dragged below the waters.
(iv) Narasimha (man-lion): to destruct the demon Hiranyakasipu who posed as God himself and prohibited the worship of Vishnu throughout the fourteen worlds.
(v) Vamana (dwarf): to reclaim the celestial kingdom from the Asura King Bali who had conquered it and driven the gods out of their native soil.
(vi) Parasurama (Brahmin): to destroy the whole race of Kshatriyas who became overpowerful and were persecuting the Brahmins.
(vii) Dasaratharama: to destruct demon King Ravan of Lanka.
(viii) Krishna: to destruct the demons Kansa and Shishupala.
(ix) Buddha: to teach heresy to the people so that, according to the Brahmins, they would be given added chances to return to Hinduism.
(x) Kalki:-this tenth incarnation is yet to come. The Hindus believe that in Kaliyuga (the present age) men will degenerate to such an extent that Vishnu will come in the form of Kalki to destroy them and build a new world.