The Ancient Igbo’s religion and tradition known as Odinaani is the form of worship practiced for over eight hundred years by the Igbo nation.

Igbo Nation in the sense that the organization of the Igbos [ibo] were somewhat like that of the ancient Greek City States. Each Igbo city or town was independent of the other with their own dialects and patron deities. They waged wars against each other and conquered each other. Basically they were the same people under different banners.
The Igbo [Ibo] religion (not traditional religion) is based on the belief of one creator known as Chukwu (the High God), Chinenke (the creator). But the Creator cannot be worshipped directly by man which brings up the other deities under the Creator. These deities known as agbala [agbara], alusi [arushi] control different spheres of human life and interactions and may have conflicting spheres.
Each Igbo community has a patron alusi [or more] and may worship several others. New alusi maybe invoked during special celebrations, war, coronations, initiations, peacetime and diplomatic meetings. Basically, the Igbos [ibo] have hundreds if not thousands of alusi.
Each deity has a priest [ezemmuo; pronounced: eeeze-mmuoo] dedicated to serving it. The ezemmuo carries out necessary functions like naming, consecration, making sacrifices and atonement. The succession of the ezemmuo is usually by his first son who starts to learn the Way of the God from childhood (around 5 years).
The Igbos [ibo] have a pantheon of deities that control life. Deities like Kamalu [popularly known as Amadioha], Igwekala, Ogwugwu, Anyanwu, Ekenwu, Mbe, Orie, Afor, Eke, Nkwo, Idemili, Otamiri, Agwu-Nsi, Agwu and a host of others control the life of the Igbos [ibo]. These deities are different manifestations of the Creator [Chukwu, Chineke] that control the forces of nature though they are not really part of Chineke.
The ezemmuo contacted the alusi through a series of sacred acts and words known as divination [áfà]. A priest divining messages from a god was in the act of ígbà áfà [act of divining]. Through divining the laws and desires of the alusi are made known and necessary sacrifices for atonement are communicated.
NOTE: A priest [ezemmuo] should not be confused with a native doctor [dibia].
The place of worship of a god (deity) is a shrine specially erected in honour of the deity. It is administered by the ezemmuo who can employ the help of his immediate family especially his successor in maintaining the place. The shrine is strictly decorated with symbols of the deity that owns it contrary to what we see in Nollywood Movies.
The most ancient forms of Igbo [ibo] religion also incorporates human sacrifices done to appease a deity during periods of great turmoil.  Human sacrifices were not carried out rampantly and have completely ceased now. Then, such sacrifices were seen as a last resort to earn the favour of the god being sacrificed to. Candidates for human sacrifices had to be ceremonially pure (which leads to the popular misconception of virgins being used for sacrifices), young but not necessarily willing to offer themselves.
The Igbo tradition also makes provision for ancestral worship. The spirits of the dead continued life in Ala Mmuo and had to be sustained by libations poured by living descendants. The dead could be beckoned to intercede for the living and protect the future generations. Like the alusi, the dead could also be displeased by the living hence the sacrifices made to keep their favour. The breaking of kolanut was done to honour the dead [ancestors].
Following the advent of Christianity in the 18th century into Igboland, the worshippers of the Igbo religion has dwindled geometrically over the years till now. Only a handful practices it and even among those who practice it many ceremonies have been lost or have lost their true significance and practices.
Sites of significance like Arochukwu, Aguleri, Owerre (Owerri), Enugu, Nkanu have been desecrated by churches and declared church-land. The religion of the Igbos were the pillar of their nation and civilization. The loss of ezi Odinaani [true Igbo tradition] has indirectly led to the division of the Igbos and the gradual loss of their cultural heritage.