Morsmordre Dark Mark
Brilliant Malfoy Manor II
Astronomy Tower, Hogwarts Castle
During the First Wizarding War, Death Eaters would cast this symbol into the sky with the Morsmordre spell, whereupon billowing green smoke would form the iconic skull and snake. This warned that an act of aggression had been committed on the premises. The spell is said to have been a creation by Voldemort himself.