Polytheistic religion developed by Indo-Aryan-speaking settlers who migrated from Central to South Asia in the second millennium BCE. It originated in the Indian subcontinent between 1500 and 1100 BCE, based on the orally transmitted liturgical works known as the Vedas, which were written down around the fifth century BCE in ancient Sanskrit. Defining features of Vedic ritual include the mandatory presence of the officiating priest or brahman, offerings made to a sacred fire, animal sacrifice (especially horse sacrifice or ashvamedha), and the use of a hallucinogenic drink known as soma. Prominent Vedic deities include Indra and Varuna; Vedic mythology features a large number of major and minor deities, demons and spirits. Vedic religion introduced the organisation of society into four castes or varnas, in which the priestly class are considered superior. There is significant similarity between the earliest forms of Vedic religion and ancient Iranian religion of the same period.