Final answer:
Alan Campbell agrees to do as Dorian asks because of a dark secret from their past.
Step-by-step explanation:
In the novel 'The Picture of Dorian Gray' by Oscar Wilde, Alan Campbell agrees to do as Dorian asks because Dorian threatens to reveal a dark secret about their past. Dorian has a portrait of himself that shows the effects of his immoral actions while his physical appearance remains unchanged. Alan Campbell was involved in one of those immoral actions and Dorian uses this secret to manipulate him into helping.