Elder Oak Tree

The Elder Oak Tree (Quercus alba) is one of Nature's eternal objects and a powerful ingredient that can be used in magic. Its wood is the only thing which can kill an Primordial vampire. It comes from the genesis of the vampire race and is the source of their immortality.

History
Morgana called upon the Elder Oak Tree in a spell to make her family immortal. As a consequence of the spell, the wood from the Elder Oak Tree could kill the Primordials. Thus they burned it but Ragnar managed to carve a stake from the tree before its destruction.

300 years after the Primordials left the New World, the natives planted a new tree and around 1912, the tree was cut down by the Sullivan family so that its wood could be used to build Wickery Bridge.

Season Three
It was revealed by Veronica that a new tree was regrown and worshiped 300 years after the Primordials fled the New World and burnt down the original Elder Oak Tree. Veronica discovered this after seeing the markings on the cave walls underneath the Richards Mansion, and she subsequently told Klaus about this.

In Break On Through, Damian finds out that the Elder Oak Tree was used to build Wickery Bridge. From this information Sage tells Rebekah and Rebekah burns down the bridge. However Damian finds a sign made from the White Oak Tree and gives it to Alaric to carve stakes from it.

Grayson Journal Entry

 * "The wood from one tree and one tree alone, an ancient Elder oak would bring death to a Primordial Vampire. When the tree burnt, all hope was thought lost. But the ash from the tree was saved and Witches forged a dagger which the ash could be bonded. This alchemic bond provides the necessary poison. It must be brandished by Humans alone for it would bring death to all Demons who wield it. As long as the dagger stays in place, the Primordial for all intents and purposes is dead."

Trivia

 * It is mentioned in the Grayson Journals that a group of witches saved the ashes of the Elder Oak Tree and created the Elder Oak Ash Daggers to bond it with.
 * According to the markings on the cave under the Richards Mansion, a sapling of the Elder Oak Tree was planted three centuries after the Primordials burnt it down.
 * In 1912, the Sullivan family cut down the sapling and used its wood to build Wickery Bridge.
 * Despite being the source of the Primordials' immortality, its destruction had not killed them in comparison to how Ellie's death killed Alaric. It appears as if the Elder Oak Tree is truly an eternal object as its power transcends death itself even after its own.
 * However, at every point in the history of the Primordials, a fragment of the tree had always been present. Ragnar had a stake carved out from the original tree that lasted until very recently. In 300 years, a sapling from the original tree was planted which ultimately yielded the wood for Wickery Bridge, subsequently ending up as 12 Elder Oak Stakes - one of which was made indestructible. So, in a way it is inconclusive whether or not somehow destroying the last piece of Elder Oak could lead to the deaths of the Primordial and the vampire race.
 * While they were children Klaus carved a wooden knight out of Elder oak and gave it to Veronica to calm her during a storm.
 * 5,000 years later it was given to Hope as a toy.
 * It has since been reshaped into seven wooden bullets by Aurora Delacourt.
 * Presumably, the reason why another Elder Oak Tree appeared in history after the Primordials destroyed the original Elder Oak tree was because of Nature needing to find balance, since it demands that everything dwelling in existence must have a mortal flaw, and that the Primordials threatened to upset that balance without the existence of the one thing that could kill them, similar to how Doppelgänger are born.