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1.
Heavy metal mixed with world music- traditional music done by indigenous folk (African, Austrailian, Middle Eastern, etc.) Instruments such as native aucoustic instruments and drums are used along side with heavy metal instruments.

World metal was most likely created and influenced by the Brazilian death metal group Sepultura (which is my most favorite metal band).

It is a very innovative type of heavy metal because they brought two completely different sounds and fused them together with nice results.
Sepultura, Soulfly, Ektomorf, etc. and even System of a Down have experimented with world metal.