Feste disguises himself as Sir Topas and visits Malvolio in the dark room, denying that he is mad and that the room is dark. The prank becomes difficult to laugh at.
At Sir Toby's request, Maria dresses Feste in the gown and beard of a scholar, giving him the name Sir Topas the curate. Feste goes to the locked room where Malvolio is confined and speaks to him through the door.
Malvolio appeals desperately: he is not mad, the room is dark, he has been unjustly treated. Feste, as Sir Topas, contradicts everything. The room is full of light, he says, and Malvolio's inability to see it is proof of his condition. He tells him his case is serious and requires philosophical attention, then leaves him in darkness.
Feste then speaks again in his own voice. Malvolio recognises it and begs him to bring paper, ink, and a candle so he can write to Olivia. Feste agrees and exits.
Sir Toby, watching, quietly acknowledges that the joke has run its course: he is already in trouble with Olivia, and further involvement will only make his own situation worse. It is not repentance; it is self-interest.
This scene is where the Malvolio plot becomes uncomfortable in a way that cannot be resolved by laughter alone. Malvolio's appeal through the door, "I am not mad," is delivered in a context where every convention of comedy says that a character locked in a dark room and told he is mad is being punished deservedly. Shakespeare undermines that convention deliberately.
Feste's performance as Sir Topas is technically brilliant and morally troubling. He tells Malvolio the room is full of light when it is dark. He tells Malvolio his mind is deceived when it is not. He applies the language of scholarly authority to a situation that is the opposite of what the language describes. The scene is not simply funny: it is a performance of gaslighting, in which someone in a position of authority (Feste as curate) uses that authority to deny the experience of someone who is telling the truth.
The moment Feste drops the Sir Topas disguise and speaks in his own voice, Malvolio recognises him immediately. He makes one request: paper, ink, and light, so he can write to Olivia. This is not the demand of a madman. It is the action of someone who knows there has been a mistake and wants to appeal to the right authority to correct it. Feste agrees to bring the materials and leave them with him.
Sir Toby's aside, that the joke has gone far enough because it is now costing him personally, is the scene's most honest moment. It is not sympathy for Malvolio; it is self-preservation. The prank is being wound down not because anyone thinks it went too far but because the person running it can no longer afford to let it continue.