Martin Eric Ain (real name Martin Stricker, born July 18, 1967 in the US) ) is best known for being the former bassist of the influential extreme metal band Celtic Frost.
 He used the stage name Martin Eric Ain throughout his career in Celtic Frost.
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.
Extreme metal is a loosely defined umbrella term for a number of related heavy metal music subgenres that have developed since the early 1980s.
 It has been defined as a "cluster of metal subgenres characterized by sonic, verbal and visual transgression".
 The term usually refers to a more abrasive, harsher, underground, non-commercialized style or sound associated with the speed metal, thrash metal, death metal, black metal and doom metal genres.
 With the exception of doom metal, all of these genres are characterized by fast tempos, attesting to their roots in hardcore punk, which has also fused with extreme metal in the forms of crossover thrash, crust punk, grindcore, sludge metal and metalcore.
 Though many extreme sub-styles are not very well known to mainstream music fans, extreme metal has influenced an array of musical performers inside and outside heavy metal.
Only Death Is Real: An Illustrated History of Hellhammer and Early Celtic Frost 1981–1985 is a non-fiction book written by Tom Gabriel Fischer with collaboration of Martin Eric Ain, published through Bazillion Points on March 30, 2010.
 The book features an introduction by Nocturno Culto of Darkthrone and a foreword by author Joel McIver.
Emperor Magus Caligula (Magnus "Masse" Broberg; born 23 May 1973 in Ludvika, Sweden) is an extreme metal musician best known as the former vocalist, bassist and lyricist of Swedish black metal band Dark Funeral, performing for the band between 1995 and 2010.
 He has also been the vocalist of death metal bands Demonoid, replacing Christofer Johnsson, and Sanctification.
 He was the original vocalist for Hypocrisy and has featured in other extreme metal bands including Dominion-Caligula (with former Dark Funeral secondary guitarist Dominion) and God Among Insects.
 He has also performed backing vocals for the Swedish black metal band Sportlov.
Patrick Bruders is an American musician, best known as the current bassist for heavy metal supergroup Down, the former bassist of the blackened death metal band Goatwhore from 1997 to 2004, and the former bassist for the seminal sludge metal band Crowbar from 2005 until 2013.
 In 2008 he joined Eyehategod side project Outlaw Order and began live bass duties for the band, but has since parted ways with the group.
 He began touring as live bassist with heavy metal supergroup Down in early 2011, replacing former bassist Rex Brown, before being added as a permanent member, performing bass duties live and in the studio.
 Bruders is also a member of the New Orleans-based crust punk band Gasmiasma, Austin-based country band Pure Luck, and joined legendary doom metal band Saint Vitus for their performance at the Hammer of Doom music festival in Würzburg, Germany.
George "Corpsegrinder" Fisher (born July 8, 1969) is an American extreme metal vocalist for the American death metal band Cannibal Corpse, the lesser-known melodic death metal band Paths of Possession, and the lesser-known extreme metal supergroup Serpentine Dominion.
 He recorded two albums with Florida's Monstrosity before leaving and joining Cannibal Corpse in late 1995, replacing vocalist Chris Barnes, who is now the vocalist for Six Feet Under.
 He also provided guest vocals for New York technical death metal band Suffocation on the songs "Reincremation" and "Mass Obliteration" from their debut album, "Effigy of the Forgotten", as well as guest vocals for California deathcore band Suicide Silence on the song "Control" from their fourth album, "You Can't Stop Me".
 Fisher also performed guest vocals on Job for a Cowboy's 2014 album "Sun Eater" on the song "The Synthetic Sea."
 As a death metal vocalist, Fisher employs the "death growl" extended vocal technique.
Death was an American death metal band from Orlando, Florida, founded in 1983 by guitarist and vocalist Chuck Schuldiner.
 Death is considered to be one of the most influential bands in heavy metal and a pioneering force in death metal.
 Its debut album, "Scream Bloody Gore", has been widely regarded as the first death metal record, while the band's driving force, Chuck Schuldiner, is acknowledged as the originator of extreme metal.
 Death had a revolving lineup, with Schuldiner being the sole consistent member.
 The group's style also progressed, from the raw sound on its early albums to a more sophisticated one in its later stage.
 The band ceased to exist after Schuldiner died of glioma and pneumonia in December 2001, but remains an enduring metal 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.
Terrorizer is an American grindcore and death metal band formed in 1986 in Los Angeles, California.
 After disbanding, its members gained recognition by playing in influential extreme metal bands, such as Morbid Angel (David Vincent, Pete Sandoval), Napalm Death (Jesse Pintado) and Nausea (Oscar Garcia, Alfred "Garvey" Estrada).
 To date, Terrorizer has released three studio albums, and broken up twice (in 1989 and 2006).
 The band reunited again in 2009, this time without Pintado, who had died three years earlier.

Question: The former bassist of what influential extreme metal band, also collaborated on the non-fiction book Only Death is Real?
Answer: Celtic Frost