The Children of Húrin

Yes -- it tells the same story as the chapter Of Turin Turambar in The Silmarillion, but in much greater detail and as a complete stand-alone narrative.

The book is dark and ends in tragedy, with significant violence, death, and themes of incest and suicide. It is intended for adult readers, or at minimum mature teenagers.

Not strictly -- The Children of Hurin is designed to be read as a stand-alone novel. However, familiarity with The Silmarillion adds depth, particularly regarding who Morgoth is and the larger context of the First Age wars.

The novel is substantially longer and more detailed, with greater characterisation, more dialogue, and a more fully realised narrative texture. The Silmarillion version is a compressed mythology; this tells the same events as a complete story.

Turin's story is closely modelled on the legend of Kullervo from the Finnish Kalevala and shares structural parallels with Sigurd the Volsung from Norse mythology -- both of whom are cursed heroes of extraordinary ability whose pride and doom are inseparable. Tolkien was steeped in both traditions as a philologist.

Yes -- Glaurung, the Father of Dragons, is the story's most dangerous villain in the second half of the narrative. His power lies not in fire but in illusion and psychological manipulation.

Tolkien identified three stories as the Great Tales of the First Age -- Of Beren and Luthien, The Children of Hurin, and The Fall of Gondolin. These three form the emotional and narrative core of The Silmarillion and were the stories he worked on longest and cared about most.