Ofermod was formed in 1996 by Nefandus drummer, backing vocalist and lyricist Mika "Belfagor" Hakola. The band recorded the Mystérion Tés Anomias single in Tore "Necromorbus" Stjerna's Necromorbus studio in January 1998 and released it through Pounding Metal Productions. In 2000, Hakola retired from his musical activities to concentrate on his occult studies and practices. In 2003, while in prison, Hakola started composing again. In 2004, he was joined by singer Nebiros again, and they recruited drummer Tore Stjerna, who had produced the Mystérion Tés Anomias single. After he served another prison sentence, Hakola got in contact with guitarist Atum, and through him with bassist Tehôm. In December 2004, the band recorded two songs for the Mystérion Tés Anomias re-release through Norma Evangelium Diaboli; on the re-release, Hakola used the new pseudonym "Michayah", and Nebiros called himself Leviathan.

The band's first album, Pentagrammaton, which was supposed to feature Moloch of Teitanblood sharing vocals with Nebiros and "mark the ten year anniversary of OFERMOD" in 2006, was "partly finished in late 2005/early 2006", but delayed because "Michayah was sent to prison, for assault, robbery and a few other incidents, in early march 2006". He was released in August 2006, but sentenced to serve more time the same year, "due to new acts of violence". Pentagrammaton was therefore further delayed until 2007. The album remained unreleased because "obviously the forces of chaos did not want this album to be released". Hakola took time to solely re-work the material, which was produced by Marduk bassist Magnus "Devo" Andersson and released under the title Tiamtü in 2008.

Ofermod is based on the forces of Sitra Achra. Hakola claims to write the lyrics in connection with meditation and astral travels and composes the music in semi-trance. Hakola tries to avoid being consciously influenced by other bands. Ofermod's musical style supposedly influenced other Swedish black metal bands such as Watain and Ondskapt. On Tiamtü, Ofermod "has eschewed breakneck speed for brutish, evocative music, which has far more technically in common with Death Metal than Black Metal". It has been likened to bands such as Watain, Dark Funeral and Gorgoroth.

The band originally worshipped the Devil and termed the music "Orthodox Black Metal". The new tracks on the EP re-release were characterised as "Orthodox Death Metal", "for the band no longer use the word 'Satan' (ha-shatan) as a description of their religion, but leave the holiest essence of the Lord silent, as it forever should be, for it existed before the cosmos and eventually will bring to its primal form: No-Thing = Utter Spiritual Death". Yet Ofermod is still connected to Satanism; Belfagor stated that "our lyrics and most of all the presence conjured into our album in the process of making it is SATANIC in every way possible", and that he considers black metal and death metal to be "exactly the same, when it comes to the Cult, as it is the same satanic forces of Chaos we religiously devote ourselves to". The band refers especially to the Kabbalah and chaos as the dark counterpart to creation, which is seen as the true evil by Ofermod, but Hakola also uses terms like evil for his own views, being evil from a cosmic perspective. According to Hakola, "[t]he materialization of existence must and will be inverted, emanation by emanation, for it is a curse life has inflicted upon itself through manifestation".