Lords of Chaos is loosely based on a novel of the same name. Very loosely. In fact, it's pretty important when you go into this film to remember those opening lines where the film is based on truth and lies; so accuracy is not exactly the goal of the film, especially due to the generally unreliable narrative from the young people involved. Most people that know heavy music would have heard the story of early Mayhem, but it's often forgotten how young they really were--guitarist Euronymous was 16 when he started Mayhem.
For those unaware of what Lords of Chaos is about, it follows the early years of Norwegian black metal band, Mayhem; a band that was highly controversial, being involved with arson, suicide, murder, and self-mutilation. Being a Hollywood film, there is a certain amount of sensationalism going on, with many liberties taken. It creates an engaging film, despite not being overly accurate (which is probably why the bands did not allow any of their music in the film).
The film focuses on guitarist Euronymous as the protagonist and has cherrypicked certain aspects and events to ensure that he comes across as likeable. Omitting a lot of the negative things he did, and inserting a love interest to the story, it makes sure to paint him as the good guy.
Strangely enough, for a film that involves very graphic murder and suicide scenes, there is a good balance of humour. There are several moments where you will have you laughing and it fits together quite naturally. While the first 20 minutes or so are rather slow with little direction, it soon picks up and the pacing improves.
One thing the film does quite well is to recreate the look and visual setting of the film. But everything is all in the looks. Perhaps it is to do with the films focus on "posers" and "fakers", but Lords of Chaos doesn't actually bother taking the time to go into what makes black metal music unique, and doesn't really look into the black metal community itself. It instead focuses on the interpersonal drama between the band members. It's rather odd that a movie about a black metal band spends so little time on the music, but I guess when you aren't allowed to actually use the band's music, your hands are tied.
Overall, it's an engaging film. It paints the whole black metal scene as a bunch of awkward angsty teenagers, which feels more at home in the punk genre. But it does make you care about the protagonist. And isn't that the whole point? As long as you aren't going into the film for an accurate documentary, there's certainly an interesting dramatic story that is worth a watch.