In my experience, Tanith Lee is an author about whom few have any ambivalence: you either love her writing or you hate it. For myself, I love it, although I'll admit that I cannot take it in large doses, as it's exceedingly rich -- "florid," some might call it -- and I find it very easy to get lost in it without any real comprehension of what I'm reading.
That's particularly true of her 1978 novel, Night's Master, which is the first in her "Tales from the Flat Earth" series. The series is so called because they take place in a fantasy realm that really is a gigantic, flat square floating within a sea of eternal chaos. Above the Flat Earth is the Upperearth inhabited by the ethereal, distant gods who have little to no contact with the mortal beings that live below. Beneath the world is the Underearth, which is the realm of demons, who, unfortunately for humans, are not nearly so distant as are the gods.
The novel itself concerns Azhrarn, who is the Prince of Demons, who functions both as its protagonist and antagonist -- such is the conflicted nature of this otherworldly being and his relationship with the human beings he at once despises and loves, as they give meaning to his existence. The book itself is a peculiar one, both in form and content. I've already noted that Lee's writing is rich, possessing a dream-like quality to it, which is perhaps fitting since Night's Master is less a traditional novel and more a collection of episodes in which Azhrarn engages in his task of inspiring wickedness in humanity. There's a dark fairy tale quality to the entire book and Lee is very good in maintaining this quality throughout. As I said, her writing is not to everyone's tastes, but there's no denying that her talent.
Compared to many treatments of Lucifer-like characters, Night's Master is much more subtle and complex. While we're clearly meant to sympathize with Azhrarn on some level, he doesn't come across as a heroic but doomed rebel, a prototype for idealistic revolutionaries striving against irrational tyrannies and I'm grateful for that. Azhrarn is a very unpleasant being and his actions are usually despicable. What the novel does do, though, is provide some insight into what drives the Prince of Demons to do what he does and it's here that I find it most interesting. Far from being a black hat from central casting, Azhrarn is a fully-realized villain -- but villain he is. Night's Master is thus an excellently mythological take on the psychology of evil, set in an interesting world and supported by gorgeous prose. Even if you don't enjoy it as much as I did, I think it's worth reading at least once.