Elder Wand

The Elder Wand, originally owned by Antioch Peverell, is one of the three Deathly Hallows created by Death himself, according to “The Tale of the Three Brothers”.  Dumbledore has a different thought about the creation of these objects. “Whether they met Death on a lonely road…I think it more likely that the Peverell brothers were simply gifted, dangerous wizards who succeeded in creating those powerful objects.  The story of them being Death’s own Hallows seems to me the sort of legend that might have sprung up around such creations.” (DH35).

• The Elder Wand is one of those creations about which, as Mr. Weasley warned, you should "never trust anything that can think for itself if you can't see where it keeps its brain" (CS18). The Elder Wand always knows who its true master is even while separated by great distances. And as Mr. Ollivander often says, "the wand chooses the wizard" (PS4). In this case the Elder Wand only chooses wizards who show some kind of mastery over other wizards, but not necessarily killing or death.
• Other names for this legendary wand include the "Wand of Destiny" and the "Death Stick" (TBB, DH21, DH32).
• The core of the elder wand is a thestral tail hair (JKR, TLC2).
• Elder wood has long been considered bad luck, according to the old saying "wand of elder, never prosper" (Pm, DH21, TBB).
• According to Dumbledore, other names for elder include "Ellhorn" and "Eldrun" (TBB).
• Historical owners include the original owner, Antioch Peverell (DH21), Emeric the Evil, Egbert, Godelot and his son Hereward, Barnabas Deverill, and the evil Loxias (possibly killed by his own mother) (TBB). According to Mr. Lovegood, either Arcus or Livius took the wand from Loxias (DH21).
• The wand-maker Gregorovitch possessed it, but it was stolen from his workshop by Gellert Grindelwald, who subsequently lost it to his old friend Dumbledore in a duel in 1945 (PS6, DH24). Grindelwald used it to great effect when escaping from MACUSA custody and attempting to set Paris ablaze in 1927 (CG).
• Dumbledore possessed the Elder Wand until his death and recognized it as one of the Deathly Hallows.
• Draco Malfoy disarmed Dumbledore with "Expelliarmus!" and received the loyalty of the wand, even though Snape performed the Avada Kedavra moments later. The Elder Wand remained loyal to Draco even after it was buried with the Headmaster (HBP27). Voldemort killed Gregorivitch in his workshop, then Grindelwald in prison, before stealing the wand from Dumbledore's grave in the white tomb. However, Voldemort was never recognized as master by the wand itself because Draco was master at that time (DH23, DH24).
• When Harry wrestled away Draco's hawthorn wand at Malfoy Manor, the Elder Wand changed loyalty again, this time to Harry (DH23). When Harry faced Voldemort in the Great Hall, the Elder Wand refused to kill him and backfired on Voldemort (DH34).
• Harry was able to repair his own holly wand with the Elder Wand (DH36). He planned to put back in Dumbledore's tomb, then someday die peacefully himself without any other wizard becoming the master of the Elder Wand.

The Dark Lord seemed to think that killing was the only power recognized by the Elder Wand, and hunted down Gregorovitch and Grindelwald to do away with all previous masters of the wand. When he finally traced the ownership back to Dumbledore and broke into his tomb, he believed he had all the power in the world.

But he was mistaken -- the Elder Wand viewed wizarding superiority in quite a different way than Voldemort imagined. As Rowling said, it wasn't so much magical ability:

"The Elder Wand is simply the most dispassionate and ruthless of wands in that it will only take into consideration strength."(PC-JKR2)

The previous 20th-century masters of the Elder wand did not have to kill to gain its loyalty, only prove their strength. Grindelwald stole the wand from Gregorovitch, then Dumbledore won it in a duel without killing Grindelwald. Draco mastered Dumbledore using "Expelliarmus," and then Harry physically wrestled Draco's wand away at Malfoy Manor and became the Master of Death, owner of all three Deathly Hallows - the wand, the stone, and the cloak.  By the time the Dark Lord decided to kill Severus Snape, mistakenly thinking he was master, the Elder Wand's allegience was to Harry alone.

Dumbledore told Harry in "King's Cross" that he planned for Snape to be master of the Elder Wand by having him perform the Killing Curse. That plan was thwarted by Draco performing the Disarming Charm (HBP27, DH35). It's quite interesting that to the ancient Elder Wand, a non-violent charm would have more weight than the Killing Curse. Perhaps the wand had grown wiser over time?

It's possible that the plan would not have worked anyway, since Dumbledore and Snape planned his death in advance, so "mastery" would have nothing to do with it. Dumbledore had injured his hand putting on the horcrux ring, and Snape had confined the poison to the hand for one year only. They both knew that Dumbledore was "done for," but even then Snape did not want to be the one to tear his own soul by killing the Headmaster (DH33). Compounding the issue, Snape also had another problem: he had made the Unbreakable Vow with Narcissa Malfoy to perform the task which the Dark Lord wanted Draco to do. So he could not hesitate to say "Avada Kedavra" on the Tower or he would have died instantly (HBP2).

Unfortunately for him, Voldemort did not know about any of these events, and after he decided that Snape was keeping the Elder Wand from working correctly, killed him using the poisonous horcrux snake, Nagini (DH32). As Dumbledore said to Harry in King's Cross, "Poor Severus." Snape was the only one killed by Voldemort for the sake of the Elder Wand who had no idea of its true nature, and who never sought it for himself.  Throughout Wizarding history there were probably other innocent bystanders like Snape, caught up in the violence surrounding the "Death Stick."