The German krautrock band Amon Düül II released their first album in 1969 (under the name Amon Düül II), where Düül came from a fictive mythology-related word, 'dyyl', created by another Canadian rock band on their album called Tanjet [REF].
The first (gratuitous) use in a metal band's name appears to have been by Blue Öyster Cult, in 1970.