As worn by real characters Edit
Less elaborate versions of the insignia are worn by Van Rijn's Muses. and 's insignia have a (only the top part of the fleur-de-lis, and the lines are more flowing). version is slightly more elaborate. Note that Van Rijn created the Muses for the Storm King, so it seems reasonable that they carry a (version of the) Storm King insignia.
As worn by fictional characters Edit
The fleur-de-lis is worn by the King, troops, Prince, Nurse, and burnt-out castle in Klaus'sabout how the Storm King became King.
In the Storm King Opera, only the muses, but not the Storm King himself, are as wearing the fleur-de-lis.
As not worn by Tarvek Edit
Although Tarvek is the present-day Storm King (via his mother's side), he has not been seen wearing the Storm King insignia. Rather, he used to wear the Sturmvoraus insignia (via his father's side), most recently . This makes sense, given that Tarvek's Storm King ancestry was at that time a closely guarded secret. At present, Tarvek does not wear any insignia. Indeed he has not worn any formal clothing, or too much clothing at all, for a .
Outside world Edit
In the real world, the fleur-de-lis is a symbol frequently used in heraldry, and often associated with France.