Bellatrix Lestrange's knife

"Bellatrix was supporting Hermione, who seemed to be unconscious, and was holding her short silver knife to Hermione’s throat. “Drop your wands,” she whispered. “Drop them, or we’ll see exactly how filthy her blood is!” Ron stood rigid, clutching Wormtail’s wand. Harry straightened up, still holding Bellatrix’s. “I said, drop them!” she screeched, pressing the blade into Hermione’s throat: Harry saw beads of blood appear there. “All right!” he shouted, and he dropped Bellatrix’s wand onto the floor at his feet."

- Bellatrix Lestrange threatening Hermione Granger with the knife

Bellatrix Lestrange owned a short knife made of silver.

History
Bellatrix held this knife to Hermione Granger's throat in the spring of 1998 in Malfoy Manor, after having tortured Hermione into unconsciousness, prompting Harry Potter and Ron Weasley to rush to her rescue once Dobby the house-elf freed them from the dungeons below. Ron disarmed Bellatrix as soon as he entered the room, prompting Bellatrix to threaten to kill Hermione if they didn't drop their wands, even drawing some beads of blood from her neck. Harry and Ron complied, but Dobby then reappeared, sending the chandelier crashing down right above the two witches. Bellatrix released Hermione in order to jump out of the way. Ron recovered Hermione from the wreckage and Disapparated, while Harry grabbed wands from Draco Malfoy and then Disapparated with Dobby.

However, Bellatrix threw her knife at them, trying to aim at Harry, as they vanished, and struck Dobby directly in the chest. The house-elf died soon afterwards, near the garden of Shell Cottage, where he was subsequently buried.

Behind the scenes

 * In Harry Potter and the Deathly Hallows: Part 1, Bellatrix used her knife to carve the word "mudblood" into Hermione Granger's arm. The film version of the knife bears a resemblance to cross-shaped spearheads that leave a hard-to-heal wound.
 * The Noble Collection sells a replica of this knife.
 * The real-movie prop, however, is much shorter, as seen in the Harry Potter Exhibition in New York City.

Appearances

 * Harry Potter and the Deathly Hallows 
 * Harry Potter and the Deathly Hallows: Part 1
 * Harry Potter and the Deathly Hallows: Part 1 (video game)