Dark Mark

From Hpwiki

The Dark Mark is the sign of Lord Voldemort, a large skull with a serpent as tongue. It is cast by use of the Morsmordre spell. When Voldemort was at the height of his power, he and his Death Eaters would cast it over a building where they have killed- the sight of it would strike fear into the hearts of the viewer. The Ministry warns the public that if they see the Dark Mark over a building, DO NOT GO IN, but to alert the Ministry immediately.

We first learn about this spell when it is cast by Barty Crouch, Jr. at the Quidditch World Cup, throwing the entire crowd into chaos and scaring the Death Eaters away.