Onslaught were formed in 1982 in Bristol, England, by guitarist Nige Rockett and drummer Steve Grice. The two musicians were working to forge an ultra-aggressive, speed metal sound that was becoming more popular in the days of early Slayer and Metallica. In 1985, the duo were joined by bass player Jase Stallard and vocalist Paul Mahoney. The quartet quickly worked up enough material for a record and later that year, the debut album, Power from Hell, was recorded and released on the British independent record label, Cor.
The band started to write more heavy metal–oriented songs, rather than their original punk sound. This was mainly influenced by the early releases of thrash metal bands, albeit with a darker sound. The band signed to Children of the Revolution Records, and released their album, Power from Hell, in 1985 as a result. As with other metal bands of the time, the lyrics were often Satanic, and the cover showed a demon emerging from a pentagram.