Heavy metal music
Critics disagree over who can be thought of as the first heavy metal band. Most credit either Led Zeppelin or Black Sabbath, with American commentators tending to favour Led Zeppelin and British commentators tending to favour Black Sabbath, though many give equal credit to both. A few commentators—mainly American—argue for other groups including Iron Butterfly, Steppenwolf or Blue Cheer. Deep Purple, the third band in what is sometimes considered the "unholy trinity" of heavy metal (Black Sabbath, Led Zeppelin, and Deep Purple), despite being slightly older than Black Sabbath and Led Zeppelin, fluctuated between many rock styles until late 1969 when they took a heavy metal direction.
Black metal
Hellhammer, from Switzerland, "made truly raw and brutal music" with Satanic lyrics, and became an important influence on later black metal; "Their simple yet effective riffs and fast guitar sound were groundbreaking, anticipating the later trademark sound of early Swedish death metal". In 1984, members of Hellhammer formed Celtic Frost, whose music "explored more orchestral and experimental territories. The lyrics also became more personal, with topics about inner feelings and majestic stories. But for a couple of years, Celtic Frost was one of the world's most extreme and original metal bands, with a huge impact on the mid-90's black metal scene".