Question: Was Oystein "Euronymous" Aarseth a guitarist in the band or co-founder of the band Mayhem?
Context: 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. 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. Chimera is the third full length album by the Norwegian black metal band Mayhem.  It is the fourth and last main Mayhem release with the vocalist Sven Erik Kristiansen (Maniac).  TurboNatas of the Norwegian band Red Harvest provided the album artwork.  The album art is a screenshot from the 1922 silent film "Häxan". 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. Abruptum is a black metal and dark ambient solo side-project band from Sweden.  It is run by Evil (Morgan Steinmeyer Håkansson) of Marduk, but the band was formed in 1989 by IT (a.k.a. Tony Särkkä).  All (a.k.a. Jim Berger), Ext and Evil joined the band later.  IT was one of the leaders of the True Satanist Horde, part of the Swedish Black metal scene.  Euronymous, co-founder of Mayhem and founder of Deathlike Silence Productions, described Abruptum as "the audial essence of pure black evil". 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. 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. 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. 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. Satan rir media ("Satan Rides the Media") is a 1998 Norwegian documentary by Torstein Grude.  The film covers the controversy that surrounded black metal musician Varg Vikernes in the early 1990s, who not only has been tied to a series of church arsons in Norway, but received widespread media attention for the murder of Mayhem band member Øystein "Euronymous" Aarseth.
Answer: He was a co-founder of the Norwegian black metal band Mayhem.