Malvolio continues to be called mad and is mocked by the members of the court—even locked in a dark room (a traditional "treatment" for mental illness at the time). During this moment, Malvolio finally confronts Olivia about the writing of the love letter. After this speech, Olivia discovers that it was in fact Maria who wrote the letter, forging her handwriting, and the Fool who was in fact the "priest" visiting Malvolio.
Malvolio's plotline makes clear the connection between love and madness, and emphasizes the nature of revelry and wildness in the environment of Twelfth Night. At the same time, it also introduces some more troubling elements to the comedy—Malvolio is certainly arrogant, dull, and hates any kind of fun, but the punishment he suffers seems to far outweigh his "crimes," and the glee the other characters derive from his suffering often feels downright cruel. While the final revelation of the play's "disguises" is a cause for happiness among most of the characters, for Malvolio it only shows him how thoroughly he has been tricked and how cruelly he has been treated—and, as he says here, for seemingly no good reason.
Why have you suffered me to be imprisoned,
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Kept in a dark house, visited by the priest,
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