Deathly Hallows Symbol – Harry Potter Lexicon

—“this is his symbol, recognized it at vunce: Grindelvald carved it into a vall at Durmstrang ver he vos a pupil there. Some idiots copied it into their books and clothes, thinking to shock, make themselves impressive—until those of us who had lost family members to Grindelvald taught them better.”
-- Viktor Krum (DH8)

An artistic portrayal of the three Deathly Hallows: a line symbolizing the Elder Wand inside a triangle representing the Cloak of Invisibility, both inside a circle that stands for the Resurrection Stone.

The symbol was associated with Gellert Grindelwald, who in his younger years along with Albus Dumbledore was obsessed with becoming "Master of Death" by owning all three Hallows at once (DH35).
Viktor Krum told Harry that Grindelwald had carved it onto the wall at Durmstrang, and that it must be Dark Magic since Grindelwald killed so many people including his grandfather (DH8).
Xenophilius Lovegood wore a triangular necklace with the Deathly Hallows symbol to Bill and Fleur Weasley's wedding (DH8). He later explained that even though Krum was alarmed because he thought the Hallows were "Dark," Xeno merely wore it to alert "other believers" who might help in the "quest" to find all three Hallows (DH20).
The symbol was hand-drawn in the copy of The Tales of Beedle the Bard bequeathed to Hermione by Albus Dumbledore. She thought it was a picture of an eye or an ancient rune, but could not find anything similar in Spellman's Syllabary. But Harry recognized it as the same shape as Mr. Lovegood's necklace (DH16).
Hermione found the symbol in Godric's Hollow, inscribed on the grave of Ignotus Peverell, Harry's ancestor from whom the Cloak of Invisibility was passed down (DH16).
In 1926, Grindelwald was in New York City disguised as Percival Graves. He gave a Deathly Hallows necklace to Credence Barebone which had a Protean Charm placed on it so Credence could contact him if he found the Obscurial who was causing havoc at that time (WFT).