Onslaught were formed in 1982/83 in Bristol, England by guitarist Nige Rockett. vocalist Jase Pope and Bassist Paul Hill and soon after to be joined by drummer Steve Grice. a demo was recorded and released later that year by this line up.

Jase Pope and Paul Hill were soon to be replaced by Roge Davies and Paul Davis respectively. The subsequent line-up recorded an EP, What Lies Ahead, in 1983.

In 1984, Paul Mahoney and Jase Stallard replaced Roge Davies and Paul Davis respectively. The band started to write more heavy metal-oriented songs 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 the band released their debut album, Power From Hell, in 1985 as a result. As with other metal bands of the time, the lyrics were often Satanic, and the now iconic cover showed a demon emerging from a pentagram.

In late 1985, Sy Keeler joined the band, who assumed the duty of members of Paul Mahoney. Mahoney retained his place in the band, but He took over bass duties from Jase Stallard, who in turn assumed the role of rhythm guitarist, giving the band a second guitarist. During the first months of 1986, the new line is ready to record their second album, The Force, in a recording studio in London. The album was released in the spring of that year through the label Under One Flag. The album was much more successful than Power From Hell, and is considered a classic by many critics and fanzines. Paul Mahoney, citing personal reasons, left the band later in 1986 and was replaced by James Hinder.

Having drawn the attention of London Records, the label signed the band. In mid 1988, the band began to start recording In Search of Sanity, their third album. Upon hearing the album, the record label felt that a more versatile vocalist was needed to do the music more justice. Although the band felt that Sy Keeler's vocals on the demo recording had the right sound, the production had a more polished sound- hence the need for a more polished vocalist. Steve Grimmett, formerly of N.W.O.B.H.M. act Grim Reaper, was drafted in to replace the departing Sy Keeler. Owing to this development, the release of the album was delayed until the summer of 1989. Following the success of The Force, and with an accomplished new singer, there was much pre-release publicity. In Search of Sanity had a much different sound than previous Onslaught releases, and many hardcore thrash metal fans were disappointed, and this partially contributed to the band's demise.

In 2004, the band reformed with the line up of Sy Keeler, Nige Rockett, James Hinder and Steve Grice. They were joined by a Welsh guitarist Alan Jordan. Writing for the band's fourth album, Killing Peace, began in 2005, and was released in early 2007.

James Hinder left the band later in 2007 and Alan Jordan left in 2008. They were replaced by Jeff Williams and Andy Rosser-Davies respectively.

In November 2008 the band recorded their live performance at the Damnation Festival in Leeds resulting in a Dual disc live album "Live Damnation" released in August 2009 by Candlelight records.

In June 2010 the band signed to the German metal label AFM records to release their 5th studio album, Sounds Of Violence, released January 28, 2011.

On 25 March 2011 Steve Grice issues a statement standing down from the band. Michael Hourihan (ex-Extreme Noise Terror / Desecration) to sit behind the drums for the forth coming tour and on November 2011 becomes a full member of the band.

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