In Hindu sacred texts, Vishnu is usually described as having dark complexion of water-filled clouds and having four arms. He is depicted as a pale blue being, as are his incarnations Rama and Krishna. He holds a padma (lotus flower) in his lower left hand, the Kaumodaki gada (mace) in his lower right hand, the Panchajanya shankha (conch) in his upper left hand and the discus weapon considered to be the most powerful weapon according to Hindu Religion Sudarshana Chakra in his upper right hand.

Vishnu is the Supreme God who takes manifest forms or avatars across various ages or periods to save humanity from evil beings, demons or Asuras. In some of the extant Puranas, and Vaishnava traditions, Vishnu's eye is considered to be situated at the infinitely distant Southern Celestial Pole.

Following the defeat of Indra and his displacement as the Lord of Heaven or Swarga, Indra asks Lord Vishnu for help and thus Lord Vishnu takes his incarnations or avatars to Earth to save mankind, thus showing his position as Supreme God to all of creation.

In the Puranas, Indra frequently appears proud and haughty. These bad qualities are temporarily removed when Brahma(one of the 3 supreme gods) and/or Shiva give boons to Asuras or Rakshasas such as Hiranyaksha, Hiranyakashipu and Ravana, who are then able to defeat Indra in wars between Devas and Asuras. The received boons often made Asuras virtually indestructible.

Indra has no option but to seek help from Vishnu. Indra prays before Vishnu for protection and the Supreme Lord obliges him by taking avatars and generating himself on Earth in various forms, first as a water-dweller (Matsya, fish), then as an amphibious creature (Kurma avatar or Tortoise), then as a half-man-half-animal (Varaha the Boar-faced, human-bodied Lord, and Narasimha the Lord with lion's face and claws and a human body). Later, Vishnu appears as human beings (Vamana the short-heighted person), Parashurama, Rama, Balarama [or Buddha according to certain schools], Krishna, and finally as Kalki for performing his task of protecting his devotees from demons and anti-religious entities.

Vishnu's supremacy is attested by his victories over those very powerful entities. It is further attested by the accepted iconography and sculptures of Vishnu in reclining position as producing Brahma emerging from his navel. Brahma the creator is thus created in turn by Vishnu out of his own person. Instead Vishnu takes various avatars to slay or defeat those demons. But it is to be noted that Vishnu also provided boons to Akupresura, a bear faced demon who was destroyed by Lord Shiva.

Vishnu's actions lowered Indra's ranking among Hindu deities and led to the ascendancy of Vishnu.

(http://indianmythcapsule.blogspot.in)