Chapter 15 in "The Picture of Dorian Gray" by Oscar Wilde is an important turning point in the novel where the character of Dorian Gray becomes fully corrupted by his hedonistic lifestyle and his obsession with his own beauty. The purpose of this chapter is to show the reader the extent of Dorian's moral decay, as he becomes more and more entangled in a life of vice and depravity.
In this chapter, Dorian is introduced to a new character, James Vane, who is the brother of a woman that Dorian had previously seduced and abandoned. James is seeking revenge on Dorian for his sister's ruin, but Dorian manages to convince him that he is not the man he is seeking. This encounter with James, however, sets off a chain of events that leads to tragedy later in the novel.
Chapter 15 also reveals the deterioration of Dorian's character and his descent into darkness. He becomes increasingly selfish and callous, and his obsession with his own beauty and youth leads him to commit heinous acts without remorse. The purpose of this chapter, therefore, is to highlight the theme of the corrupting influence of pleasure and beauty, and to foreshadow the tragic consequences of Dorian's actions.