Doppelgänger

Doppelgängers, also known as Shadow-Selves, are a supernatural occurrence that were created by Nature as result of Silas and Amarabecoming true immortals after consuming Qetsiyah's immortality elixir. The purpose of this supernatural species is to maintain the natural balance by dying in place of their immortal progenitors. For this reason, doppelgängers have existed for at least 1,000 years during which time they have been born and died in the place of Amara and Silas, who, as true immortals, were invulnerable to any conventional weapon.

There are two known doppelgänger lines which stem from Silas and Amara, respectively. Known doppelgängers of Silas are Stefan Salvatore and Tom Avery, and known doppelgängers of Amara areTatia, Katherine Pierce and Elena Gilbert.

Introduction of the Doppelgängers
The first doppelgängers introduced in this universe were Elena Gilbert and Katherine Pierce (also known as Katerina Petrova), but they were initially believed to exist due to Elena being one of Katherine's descendants. However, it was later revealed that, while doppelgängers do have a bloodline connection, they are not simply a supernatural species like werewolves-- they are actually mystically occurring beings who are loopholes to the creation of the Immortality Spell by Qetsiyah in the 1st century BCE. Because Nature decreed that all living things must die, the Immortality Spell disrupted the balance Nature had created, which forced Nature to counter it by creating living "shadow-selves" of the immortal Silas and Amara who could die in their place. These doppelgängers are believed to either be direct descendants of Silas and Amara's respective bloodlines, or descendants of one of their siblings, who would be the closest genetic match to continue their lineage.

Because their creation was a consequence of the use of magic, doppelgängers are supernatural, and as such, their blood can be used as a binding agent for powerful spells, such as the spell that created the Originals, and therefore the creation of the vampire race. However, they have no actual supernatural powers of their own aside from the mystical properties of their blood, and the only reason it is known that they are truly supernatural is because Elena mercy-killed Alaric Saltzman while he was wearing a Gilbert ring in hopes that it would cause him to come back, as the ring can only resurrect the wearer if they die at the hands of a supernatural being, and they were unsure of who had originally seriously wounded Alaric before Elena got there. Since the ring brought Alaric back from the dead, it was confirmed that Elena, despite being otherwise human, was a supernatural being. This also confirmed Elijah Mikaelson's suspicion that the Gilbert ring wouldn't work on Elena because she was technically a supernatural being. Though doppelgängers are supposed to represent mortality in the place of an otherwise truly immortal being, they can achieve immortality through vampirism, as was in the case of Katherine, Stefan, and Elena, as a result of their being technically human as well. This could also be because vampires aren't fully invulnerable creatures like immortals are-- they can be killed through various means such as staking, heart extraction, decapitation, etc.

Creation
The phenomenon of the shadow-self occurs to counterbalance a disruption to Nature's law that all living things must die. Qetsiyah, one of the most powerful witches and member of a very gifted people called the Travelers, created a spell for immortality with the intention that she and Silas would drink it together at their wedding. However, Silas stole the immortality elixir and used it on himself and his lover, Amara, who was Qetsiyah's handmaiden, so that the two could be together for eternity. Disrupting the balance of Nature when they became truly immortal, Nature eventually began to create copies of Silas and Amara that could die to counter the balance that was disrupted by these new immortals. With Silas and Amara now both dead, it is unknown if Nature will continue to spawn more doppelgängers.