Is this a quote from something? I don't understand this sentence.
Yes, this is a reference to "The Lord of the Rings" movie series. The quote from the movie is "one ring to rule them all" because the ring has magic powers, but here he jokes that it is "one mask to rule them all" as if his face is a mask with magical powers. "My precious" is another quote from the movie said by a character named "Gollum" who had become deformed and ugly-looking because of the power and corruption of the ring. He says this quote to make fun of the boy's facial deformities.