The late Vedic age saw the compilation of the two great ITHIHASAS (epics), the RAMAYANA and the MAHABHARATA. The legends of both which Ramayana and Mahabharata consist have come down, by tradition, from the earliest period of the Vedic age. The stories of these epics are secular in nature but they not only describe the feats of their heroes but also refer to the influence that the gods had on their exploits. Thus the stories of the gods were supplemented and expanded as they were woven into the narratives and the heroes themselves got assimilated into Indian popular religion and became deified.
Further development of the Indian society brought about changes in religious concepts and an increase in the size of the pantheon. This grew by a process of absorption and combination, adopting popular deities into a sophisticated and well-developed assembly and merging several deities into one. Thus the minor Vedic deity Vishnu was identified with Vasudeva and another epic hero Krishna.
Later, Krishna himself got assimilated with a pastoral flute-playing deity and became the subjects of many poems and legends. At the same time, an ancient fertility god, Shiva, was elevated to the higher ranks of the pantheon and became an important deity with a variety of forms that gave him popularity equal to that of Vishnu. Shiva and Vishnu were visualized as forming a triad with Brahma.
Eventually the traditional legends, myths and tales were incorporated into the PURANAS summing up all that was to be known about the gods. The word PURANA means "old" and the Puranas wove same historical legends and mythological fictions as the VEDAS and ITHISASAS. But they give a more definite and connected representation of the cosmogony and mysticism of these poems, and they expand and systematize their chronological computations and genealogies. They reduce the formless and fleeting religious conceptions of the Vedas and the popular family traditions of the Ithihasas to a fixed body of definite mythology.
The popular heroes of the Vedic age were transformed into Gods and the shadowy gods of the Vedas gradually took the positive forms under which they appear in the Puranas and have been worshipped since. In the Puranas the gods assumed a substantial shape and individual character. The sacrificial rites and observances of the worship of the gods, for the first time were given a paramount place.
The concept of TRIMURTI – the PURANIC GODS – emerged and gained importance over the centuries. BRAHMA the Creator, VISHNU the preserver and SHIVA or MAHESHA the destroyer came to denote the three characteristics of GOD – Generator, Operator and Destructor and the basis of the HINDU TRINITY.