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.
Extreme metal
The British band Venom are one of the first bands to venture into extreme metal territory, due to their ideological shift into themes of evil, the devil and hell. Their first two albums, "Welcome to Hell" (1981) and "Black Metal" (1982), are considered a major influence on thrash metal and extreme metal in general. This early work by Venom, in combination with bands like Discharge, The Exploited and Amebix as well as American hardcore punk brought integral elements into the budding extreme metal landscape at the time.
In 1983, Metallica would release their debut album "Kill 'Em All", which fused elements of the new wave of British heavy metal with hardcore punk and the style of Motörhead, becoming the first thrash metal album, and would eventually be certified triple platinum. A few months later, Slayer would release their own thrash metal album "Show No Mercy", influenced by the sounds of Venom, Judas Priest, Iron Maiden, and Mercyful Fate.