So there is actually quite a story behind this. Mayhem was meant to be the most extreme band that ever existed when it was originally formed, which is quite a bold statement to make really.
The lineup on this album has quite a list of people. With Hellhammer on Drums, Euronymous on Guitar, Varg Vikernes (a.k.a. Count Grishnackh) on bass, and Attila Csihar as vocalist, the band recorded what is considered by many to be one of the best black metal albums ever.
Funny thing though, the band has quite an issue with death in it's early years with their longest lasting vocalist (at the time) named Dead killing himself before the recording of this album, and Varg stabbing Euronymous to death after the recording.
Though Euronymous' family requested that the bass tracks be re-recorded on the album, Hellhammer thought it would be appropriate to keep the killer and the victim together forever. Though originally telling the family that it was changed, he kept it on and only revealed it at a later date.
With the exception of one other band, these guys are probably the most "evil" band I can think of. Not due to the killing or anything, but the content. The albums name literally translates as "Lord Satan's Secret Rites" as a prime example.
Having listened to it, this is actually one of my favorite metal albums I've heard in a while. Remember though, don't worship the devil kids.