Question: Was Oystein "Euronymous" Aarseth a guitarist in the band or co-founder of the band Mayhem?

full_context: Document 1: Dawn of the Black Hearts: Dawn of the Black Hearts is a bootleg live album by the Norwegian black metal band Mayhem. It is infamous for bearing a photograph of Mayhem's singer, Dead (Per Yngve Ohlin), after his suicide in April 1991. The photograph was taken by Mayhem's guitarist, Euronymous (Øystein Aarseth), when he discovered the body.
Document 2: Olve Eikemo: Olve Eikemo (born 27 June 1973), better known by his stage name Abbath Doom Occulta or simply Abbath, is a Norwegian musician best known as a founding member of the black metal band Immortal. Before founding Immortal, Abbath performed with Old Funeral alongside future Immortal member Demonaz. While working with Old Funeral, he also joined Demonaz's band Amputation which later became Immortal. He also plays bass in Demonaz's eponymous project.
Document 3: Rune Eriksen: Rune Eriksen (born 13 January 1975) is a Norwegian Spellemann-award winning musician and composer. He is best known as the former guitarist in the black metal band Mayhem under the stage name Blasphemer, which he took from a Sodom song. He joined the band in October 1994, but departed in late 2008. He currently plays in: Aura Noir, Earth Electric and the multinational band Twilight Of The Gods. Rune has also done guest appearances with Absu, Negură Bunget, and Root. Former bands and projects: Nader Sadek, Mezzerschmitt, and the Portuguese Gothic Doom band Ava Inferi. He resides in Portugal.
Document 4: Pentagram Chile: Pentagram Chile (previously known as Pentagram) is an extreme metal band from Chile, formed in 1985. They were part of the first wave of extreme metal in the mid 80's, along with bands like Possessed, Celtic Frost, Sepultura and others. Considered as a very influential band in the Black metal and Death metal genres, they have influenced bands like At The Gates, Dismember, Napalm Death and Avulsed among others.
Document 5: Parabellum (band): Parabellum were a Colombian extreme metal band from Medellín active in the 1980s. The band was described by "Terrorizer" magazine as one of the world's first black metal bands, as well as the first extreme metal band from Colombia and one of the very first from all South America. According to writer Emilio Cuesta, Øystein "Euronymous" Aarseth—a musician associated with the early Norwegian black metal scene—has said Parabellum and Medellín's Reencarnación were both influential to his own band Mayhem.
Document 6: Mayhem (band): Mayhem is a Norwegian black metal band formed in 1984 in Oslo. They were one of the founders of the Norwegian black metal scene and their music has strongly influenced the black metal genre. Mayhem's early career was highly controversial, primarily due to their notorious live performances, the 1991 suicide of vocalist Per Yngve Ohlin ("Dead") and the 1993 murder of guitarist Øystein Aarseth ("Euronymous") by former member Varg Vikernes ("Count Grishnackh"), of Burzum.
Document 7: De Mysteriis Dom Sathanas: De Mysteriis Dom Sathanas is the first full-length studio album by the Norwegian black metal band Mayhem. Songwriting began in 1987, but due to the suicide of vocalist Per "Dead" Ohlin and the murder of guitarist Øystein "Euronymous" Aarseth, the album's release was delayed until May 1994. "De Mysteriis Dom Sathanas" is widely considered one of the most influential black metal albums of all time.
Document 8: Obús: Obús is a Spanish heavy metal band created in Madrid in the early 80s. They stood out because they took the risk of making a big spectacle in their concerts in order to reach to the level of other international heavy metal bands. In addition, the lyrics they write and the feelings they transmit connect with the people that follow them. As they said in an interview: "All our songs , some more and some less, have a meaning". Far away from renouncing to their genre, they have always claimed that they were a heavy metal band and they even wrote a song about this genre in their first LP called"Dosis de Heavy Metal".In 2012, because of their 30th anniversary, they did a Tour around Spain.
Document 9: Black Metal (Venom album): Black Metal is the second album by English heavy metal band Venom. It was released in November 1982, during the great flourishing of metal music in the UK that was the new wave of British heavy metal, and is considered a major influence on the thrash metal, death metal and black metal scenes that emerged in the 1980s and early 1990s.
Document 10: True Norwegian Black Metal – Live in Grieghallen: True Norwegian Black Metal – Live in Grieghallen is Gorgoroth's first full-length live album, released by Regain Records. It was recorded live in the studio at Grieghallen Lydstudio in Bergen and Threeman Recordings in Stockholm. The songs on the album represented the most commonly played songs in Gorgoroth's live set. The cover design by Magnus Wohlfart was radically different from the previous three full-length albums, being more reminiscent of the cover of the album "Under the Sign of Hell" (1997). Most of the album was recorded before the departure of vocalist Gaahl and bass guitarist King ov Hell, who had intended on using the band's name and assets. The bass guitar for this album (originally performed by King ov Hell) was later re-recorded by guitarist and band founder Infernus. Infernus dedicated the album to the late Jon Nödtveidt of Dissection.
Document 11: Paradox (British band): Paradox was a Christian black metal band founded in London, England, in 1996 by guitarist and vocalist Michael, using the pseudonym "John Tarantula". They were reported to be, at the time, the only dark metal band in London promoting Christianity. Originally formed as a three-piece family unit, they released a demo "The Outcasts" in 1997 through their own record label, Tarantula Promotions. In 1998, a fourth member joined as a vocalist, and a second demo was released in 2000 entitled "Through Pain There Is Joy". Following this release, the band reformed around Michael as a five-piece unit, and a sixth member later joined part-time on keyboards. In 2002, Paradox disbanded, releasing their last two songs as part of a compilation album entitled "Overcome or Burn Forever in Hell/Arachnid Terror Sampler", which featured tracks from fourteen other artists. Following the breakup of the band, the projects Bloodshed and Slimegem were formed, and a third band, Hamal 'ak Hamashith, was promoted through Tarantula Promotions.
Document 12: Varg Vikernes: Varg Vikernes (] ; born Kristian Larsson Vikernes, 11 February 1973; legally Louis Cachet) is a Norwegian musician and writer. In 1991, he founded the one-man music project Burzum, which is considered one of the most influential black metal acts. Three years later, he was convicted of murder and arson, and subsequently served over 14 years in prison.
Document 13: Mortem: Mortem is a death metal band formed in 1986 in Lima, Peru. They are the first Peruvian metal band to have toured Europe and the United States. Mortem’s musical style is firmly rooted in the death metal, black metal and thrash metal of the nineteen-eighties. Mortem's characteristic heavy sound is defined through slow and sinister death metal alternated with fast and aggressive counterparts. Frantic tremolo picking in the style of early Possessed and Death built on passionate rhythmic percussion topped with wailing atonal and melodic guitar solos comprise a potent mix which lives up to the principle of death metal technique to craft pounding ritualistic and spirited death metal. Vocals are forged from guttural rasps and often develop into high shrieks while lyrics deal mainly with the occult and anti-Christianity.
Document 14: Necrobutcher: Jørn Stubberud (born April 13, 1968) is a Norwegian musician. He is best known as the bassist in the black metal band Mayhem under the stage name Necrobutcher. He is one of Mayhem's founding members, along with Euronymous, Manheim and . He is the band's only remaining original member, as Messiah and Manheim left Mayhem, respectively, in 1986 and 1988, and Euronymous was murdered in 1993.
Document 15: Euronymous: Øystein Aarseth (] ; 22 March 1968 – 10 August 1993), who went by the pseudonym Euronymous, was a Norwegian guitarist. Euronymous was a founder of and central figure in the early Norwegian black metal scene. He was a co-founder of the Norwegian black metal band Mayhem. He was also founder and owner of the extreme metal record label Deathlike Silence Productions and record shop Helvete.
ground_truth: He was a co-founder of the Norwegian black metal band Mayhem.