The Phantom's Face
You can never show the Phantom's face, or more specifically, his eyes.
A favourite Old Jungle Saying is "He who looks upon the Phantom's face will die a horrible death". This has come to coincidental fruition for antagonists in a number of stories, reinforcing the myth.
The Phantom's immediate family (wife and children) and a very select few others are permitted to view the Phantom's face - no one else can.
Note that on the occasion that someone rescues or helps the Phantom when injured, for example, the curse is not fulfilled.
Even when the mask has been removed for the purpose of the story, the Phantom's eyes must not be shown to the reader. They either need to be in shadow, covered or wearing glasses / sunglasses. When around family they may be in a position to see his face, but the eyes must always be hidden from the reader.
An artistic quirk that phans enjoy spotting is the moment an incoming Phantom stops having his eyes visible in the story, usually at around the swearing of the Oath.