Genre Popularizer: For black metal. Specifically speaking, Mayhem essentially started up second-wave black metal, but first-wave black metal covers a lot of bands that are totally dissimilar to Mayhem (Celtic Frost and Mercyful Fate, for example), and the actual black metal genre didn't really have a proper definition until Mayhem and co. came along. (It is worth noting, however, that many of the second wave's most important characteristics had already been established by Bathory, particularly on Under the Sign of the Black Mark).
There's vocalist Messiah, who performed on their first live show, but couldn't join the band on a permanent basis. He also appeared on two songs from their EP Deathcrush ("Pure Fucking Armageddon" and a cover of Venom's "Witching Hour"), but only because Maniac couldn't do them himself. According to Necrobutcher, when Maniac left the band in 2004, Messiah asked to join, but was rejected.
Manheim is definitely nowhere near as remembered as Mayhem's drummer as Hellhammer is.
When the band started, they actually also had a second guitarist named Per Nilsen.
Short-Lived, Big Impact: Dead's only appearance on official Mayhem releases was on Live at Leipzig (and if you wanna go more in depth, the "single" for "Freezing Moon") but yet his presence lingers within the band and arguably all of second-wave black metal. Unfortunately as believed, his suicide kick-started the eventual dark moments of the scene, from the church burnings to Euronymous's eventual murder by Varg Vikernes.
There were at least nine different bands named Mayhem worldwide that were making demos in the 80s. This lead to Norway's Mayhem to declare themselves "The True Mayhem", which they still use for websites, publicity, and their logo (the little scribbles over the Y and H).
On YouTube, you will find two different artists named Mayhem; this band, and one of the many, many alias of electronic musician Renard, represented by a shark girl dressed like a raver. The latter's videos have commentators confusing her for the former. Naturally, on sites where the two artists are combined onto one page, there are inevitable Flame Wars.
Troubled Production: If you haven't guessed from reading the article on the main page yet: Yes, De Mysteriis Dom Sathanas had one of these.
For a big general idea for those wondering: Lyrics were being written in 1987 by Dead and these songs would later be performed live, in particular the one at Live at Leipzig in 1990. Then in 1991, Dead kills himself. Via throat/wrist slitting and then shotgun to the head. Necrobutcher left afterwards when Euronymous took photos of Dead's body (which would later be used for the infamous Dawn of the Black Hearts bootleg). 1992 rolls around with recording beginning with newly recruited members Attila Csihar as vocals and Varg Vikernes on bass. The year later, Vikernes murders Euronymous. 1994 comes in and drummer Hellhammer releases the album as it is (despite Euronymous's parents asking him to remove Vikernes's bass lines but not doing so). Troubled Productiondoesn't even describe it.
What Could Have Been: Following the murder of Euronymous at the hand of Varg Vikernes, Hellhammer and returning bass guitarist Necrobutcher initially considered replacing Euronymous with Blackthorn (Snorre Ruch), who was rehearsing with the band during pre-production for De Mysteriis Dom Sathanas and also made some songwriting contributions to the album (though he did not play on the record itself). Ruch, however, was convicted as Vikernes's accomplice in the murder and was sentenced to eight years.
For the film:
Enforced Method Acting: During Derek Cho's video explaining the Reed case to the Nine and Towers, there are a pair of workers having sex in the background. The workers were in fact, an actual couple. As for the sex, it wasn't fake.
What Could Have Been: Derek's surname was originally "Saunders", but it was changed to "Cho" presumably after Steven Yuen's casting.
