What happens in the opium dens?

The opium den is an important symbol in The Picture of Dorian Gray. In Victorian England, opium was perfectly legal but every bit as deadly as its notorious derivative, heroin. The taking of opium, though widespread, still had a faint whiff of scandal about it, being considered somewhat debauched. So a number of opium dens sprung up in London to cater for the exotic tastes of a thriving demimonde of bohemians and thrill-seeking aristocrats. Wilde neatly sums up the lurid attraction of these dens of iniquity and the effect they had upon their wretched patrons:

There were opium-dens, where one could buy oblivion, dens of horror where the memory of old sins could be destroyed by the madness of sins that were new.

Dorian Gray certainly wants to forget his sins, as he has just murdered Basil. His conscience is starting to get the better of him; he needs to submerge it far beneath a drug-induced haze. So he takes off to an opium den in a remote part of town. The den symbolizes the growing degradation of Dorian's tortured mind. He wants to get away from himself, to become immersed in wholly unfamiliar surroundings where he can forget about everything.
But he can't. For one thing, Dorian discovers Adrian Singleton at the opium den, and this bothers him. Adrian is an unpleasant reminder of his past, a past he desperately wants to forget. His hopeless addiction to opium also gives Dorian an unwelcome portent of his own future. The dubious pleasures of opium cannot give Dorian what he wants. The opium den, like the picture of the title, provides us with a horrifying glimpse into a rotting soul, mired in moral corruption and utter degradation.