But nothing's ever immortal. I mean, not really. Nature won't allow it. There's always a loophole.
HOPE MIKAELSON TO LANDON KIRBY IN THERE'S ALWAYS A LOOPHOLE
Loopholes are understandable ways for Nature to balance out the existence or the excess of the Universal Lifeforms, and there is almost always a loophole for Nature to use in all magical creations. This is especially true with regards to immortality, as Nature has always decreed that all living things must die. They can also be natural occurrences created by Nature to serve as balance to the existence of something that goes against its laws. For example, the majority of the Mikaelson Family were turned into undead vampires, which went against the will of Nature; in order to balance this unnatural creation, it needed to have a weakness, and so Nature created one way for the source of the Original vampires' immortality, power, and continued existence to be used against them by taking it away.
These magical loopholes are not only used as balances, as Hope Mikaelson's conception was not only due to a loophole, but her existence as the first living hybrid with vampire blood and the first true witch-werewolf-vampire hybrid itself also causes a loophole in the Natural Order, though it's unknown what those consequences could be. Another example of magical loopholes is the law that powerful, long-lasting spells (either in the form of supernatural beings, enchanted objects, or mystical dimensions) must be bound to something that possesses similar or equal power and longevity. An example of this is the Other Side, a purgatory dimension for supernatural creatures that existed for over two thousand years due to Qetsiyah binding the spell that created it to the first immortal woman, Amara, who could not be killed by any conventional means.
Known loopholes in Nature include, but are not limited to: witchcraft, doppelgängers, the ancient white oak tree(s), the cure to immortality, Niklaus Mikaelson's ability to procreate and Hope's birth, and many more.
Throughout The Vampire Diaries Series
Season Two
The existence of doppelgänger and the fact that their potent mystical effects of their blood could be used to bind powerful spells (such as the Hybrid Curse that was placed on Klaus) was first explained. Elena was revealed to be a doppelgänger descendant of Katerina Petrova, a 500+ year-old vampire and fellow doppelgänger. At the time, the supernatural "shadow-selves" were unknown as being anything more than a natural occurrence that possessed powerful blood with immense magical potential. The Original witch was also revealed to have been forced—both by the Spirits of Nature as well as by her own husband, Mikael—to bind Klaus's werewolf aspect by using the blood of the first Petrova doppelgänger, Tatia. As a result, Petrova doppelgänger blood as the binding agent became a loophole to the spell and to Nature's will, as it was the one substance that could break the Hybrid Curse and return Klaus to his true hybrid heritage, should the doppelgänger be killed by him in a sacrificial ritual.
Season Four
In Graduation, it was revealed that when Silas ingested the immortality elixir, he became truly immortal, as in unable to be killed by any weapon or conventional means know to man. Nature's only way of getting around this spell was by creating versions of him that could die - doppelgängers of the newly-turned immortal Silas and his soulmate Amara. As a result, the male and female doppelgänger bloodlines were created from their living relatives and their subsequent offspring. This eventually spawned the births of many doppelgängers of Amara and Silas over the next two thousand years, including Tatia, Katerina Petrova, and Elena Gilbert from Amara's bloodline, and Stefan Salvatore and Tom Avery from Silas' bloodline which means only one could survive will the other dies.
Season Five
It was revealed that, when combined, the blood of the last surviving doppelgängers in each bloodline—in this case, Stefan and Elena—could be used as a loophole to break the Travelers' Curse that was cast upon the Traveler community by the earliest incarnation of the Gemini Coven in the first century BCE in punishment for Qetsiyah's creation of the Immortality Spell. To do this, the Travelers, led by Markos, performed the Magic Purification Spell using large amounts of Elena and Stefan's combined doppelgänger blood that effectively nullified all non-Traditional Magic in a given vicinity, which included the powers of supernatural beings (and, in the case of vampires, their undead lives themselves), talismans, and magical objects that were created by any form of magic that was not Traditional Magic, such as Spirit Magic, Ancestral Magic, Dark Magic, etc.
In Promised Land, the Travelers successfully performed the spell, which broke their curse and allowed them to no longer rely on Traveler Magic to survive, as they were once again able to tap into the earth's magic after millennia of being mystically barred from doing so. However, even despite the fact that the spell was halted when Stefan was killed (albeit temporarily), its effects still managed to cover the entirety of Mystic Falls, forcing werewolves, witches, and Brotherhood of the Five hunters to relinquish their supernatural abilities upon walking across its borders. Vampires and hybrids were unable to cross the borders at all without risking being stripped of their vampirism and returning to the state they were in prior to being transformed - a corpse. However, this magic purification spell was eventually broken, allowing the return of the supernatural population to Mystic Falls.
Season Six
Throughout the series, there was a recurring fundamental law of Nature that came into play: witches, as servants of Nature, were unable to become vampires without relinquishing their connection to magic. As a result, any witch turned into a vampire would immediately cease being a witch upon waking in transition. However, at the beginning of Season Six, Josette Parker revealed that there was even a loophole to this law of Nature. Over the years, a type of witch has developed in the ranks of the Gemini Coven called a Siphoner, who do not possess their own magic and instead can only practice witchcraft by siphoning magic from another source, such as a supernatural being or an enchanted object. Because they don't possess their own magic, the Siphoners fall into a loophole in Nature's law that allows them to retain their ability to siphon magic from outside sources even after being transformed into a vampire and completing their transition by consuming human blood.
In addition to retaining their ability to siphon magic, these witch-vampires hybrids are also able to practice witchcraft at will by channeling the magic from their own vampirism/immortality, essentially making their bodies a limitless source of magic from which they can draw power. In the late 18th century, a group of six Siphoners, known as the Heretics by the Gemini Coven, were exiled by the coven leader because of their unnatural abilities. Some time later, they met the vampire and Ripper Lily Salvatore during their travels, who ultimately turned them into the first (and possibly the only) true witch-vampire hybrids. In 2013, the Siphoner and Gemini Coven leader Malachai Parker also became a hybrid after he convinced Lily to turn him into a vampire; he wanted to become a vampire so he could kill himself and thereby kill the rest of his coven along with him while still being able to live and practice magic as an undead vampire, and Lily agreed on the condition that he reunite her with the rest of the Heretics. As a hybrid, Kai was not only able to channel the power of his own vampirism to perform powerful spells, but could also heal himself of a werewolf bite—an affliction that is typically fatal to a vampire without Klaus' blood as a cure—by siphoning the magic in the werewolf venom coursing through his veins and using it offensively against Bonnie Bennett and Damon Salvatore.
Throughout The Originals Series
Obviously, one of the most important loopholes in The Originals is Niklaus Mikaelson and Hayley's daughter Hope Mikaelson. Both Klaus and Hayley thought it was impossible for Hayley to be pregnant with Klaus' child due to vampires being unable to procreate, but Sophie Deveraux was convinced it was a magical miracle baby, as her conception was made possible by one of Nature's loopholes. Shortly before Hope was born, it was discovered that she was the first ever alive (rather than undead) werewolf-witch-vampire hybrid, due to inheriting her werewolf gene from both of her parents and gaining her witch and vampire heritage from her father, Klaus, whose mother was a powerful witch and whose biological father was a well-respected Alpha of one of the original werewolf bloodlines. Hope's blood also creates another loophole, as she is able to turn werewolves into vampire-werewolf hybrids by feeding them her blood before they die and after they awaken in transition; before this discovery, the only known way to create more of these hybrids was to feed a werewolf Klaus' blood prior to their death and then feeding them the blood of a human Petrova doppelgänger after they awaken.
One of the most surprising loopholes of season three is in regard to the Serratura. The laws of the Serratura state that it forms an impenetrable barrier, that cannot be penetrated by anything living or dead. Due to the fact that a vampire in transition is neither dead or alive, they are able to void the barrier and pass freely through.
Throughout Legacies Series
Landon got all the answers, but, basically, Malivore was created by the blood of a werewolf, a witch, and a vampire. It used to be that only they could destroy what they created, but nature found a loophole. By making me, the tribrid. I'm the loophole.
HOPE TO ALARIC
In There's Always a Loophole, it was revealed that Hope Mikaelson was the loophole behind Malivore's creation. Since no being can be truly immortal, Nature must always find a loophole. Hope realized that since the blood of a witch, werewolf and vampire created Malivore, only the combined blood of the three could kill him. This explains why Hope was able to remember the beings thrown into Malivore and why the bullet fashioned from his body did not harm her and her blood cured those that was struck with it.
Trivia
Loopholes only occur when something should not happen in nature, such as immortality. Witch laws state that everything should die.
At first glance, it would seem that this law contradicts itself with the example of the spell cast by Dahlia to keep her and her niece immortal with no side effects. However, the loophole is that both must slumber for a century after being awake for just one year.
Loopholes to spells do not always happen immediately after the spell is cast. The loophole to Malivore's creation occurred centuries after the spell was cast.
It has been suggested by The Originals' writer, Carina Adly MacKenzie, that loopholes can in fact be closed with spells - this is in the case of Klaus' ability to procreate as the Original Hybrid. Carina states that they once had a line that said "Jane-Anne closed the loophole with a spell", but it was cut. Whether or not Loopholes can be closed with magic remains unknown as no such spell has yet to occur.