MagicPhenomenon
Deathly Hallows
The Deathly Hallows are three objects from Beedle the Bard’s tale: the Elder Wand, the Resurrection Stone, and the Cloak of Invisibility. A mark of a triangle, circle, and line stands for them. Xenophilius Lovegood believes whoever unites the three becomes Master of Death. Harry gathers them by the end, and he treats that mastery as the willingness to meet death rather than as a throne.

First mention
Harry Potter and the Deathly HallowsChapter: The Tale of the Three Brothers
Luna’s father tells the tale at his house and shows the sign, after Harry has already seen it in other places without knowing the story.