What religion believes in one major deity who takes many forms?

Most Eastern Religions believe in this to some extent.

Some sects of Hinduism believe that all of the gods are manifestations of Krishna, the Lord of Love. This is demonstrated in the Bhagavad Gita.

In Taoism, hundreds of gods are worshiped (a celestial kingdom) as well as the Tao, a mystical force which pervades all of existence. This force is the deep joy you feel when hearing a song you like for the first time, for example.

Mahayana Buddhism also believes that Bodhisattvas are "ones who have attained nirvana" following the Buddha, in a sense they are manifestations of what the Budda has achieved.