Fiendish creatures dwell in the infernal planes, realms of evil, although they resemble beings found on the Prime Material Plane. They are more fearsome in appearance than their earthly counterparts. Fiendish creatures are often mistaken for half-fiends, more powerful creatures that are created when a fiend mates with a non-Celestial creature, or through some foul infernal breeding project.
Reference: D&D 3rd Edition Monster Manual I, Pg. 211
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