Gellert Grindelwald's sign

Grindelwald's Sign is believed by some, such as Durmstrang Institute student Viktor Krum to be a mark of evil, or Dark Magic.

The sin was later relieved to be the same sign as the Deathly Hallows.

Seen
The mark, appearing to be an eye enclosed by a triangle has been seen in many different places.

(as believed by Viktor Krum
 * On the walls of Durmstrang Institute carved there by Gellert Grindelwald himself as a student
 * As a pendant on Xenophilius Lovegood during Bill Weasley and Fleur Delacour's wedding