Moonlight Amulet

A Moonlight Amulet is a piece of jewelry with a black kyanite gemstone enchanted by witches to allow werewolves to control their transformation, thus sparing them the need to turn every month on the full moon. Their vampire counterparts are the Daylight Amulets, which prevent vampires from burning in sunlight.

It was first believed that in order for moonlight amulets to function properly, they have to be enchanted by a witch on the night of a full moon. However, it was later shown that the witches under Esther's control did not need the full moon for their amulets to be effective.

The first batch of moonlight amulets made in the modern day was crafted by Genevieve, who led Niklaus Mikaelson to believe that she was making them for Hayley Marshall and the rest of the Crescent werewolves. However, it was revealed that she had actually double-crossed Klaus and tricked him into allowing her to use his blood to bind the spell, since he can control his own transformation as a hybrid; because of this, the wearer of the amulets is given the power by drawing on Klaus' own strength, weakening him whenever a werewolf calls on the stone to prevent their transformation on a full moon. Genevieve also betrayed Klaus a second time by giving the stones to the newly-triggered Guerrera Pack, led by Francesca Guerrera. After killing all the werewolves who possessed the rings Genevieve created, Klaus' strength was no longer drained from those who wore them. The stones were then completely destroyed by Elijah, who used the elements of fire and water to dissolve the enchantment for good.

After her resurrection by possessing the Harvest girl Cassie's body, Esther began supervising the mass production of her own brand of the rings by the French Quarter Coven witches under her control, as she originally invented the spell for the stones, and did not need Klaus's blood for her rings to work. Also, unlike their predecessors, the stones were placed on silver rings, which is ironic because werewolves in legend grow weak from silver. However, in reality, werewolves heal from wounds caused by silver, and the choice of setting was likely based on style rather than function, as Elijah Mikaelson joked that the original batch of moonlight amulets, which were set in gold rings, were "gauche" like the Guerrera werewolves who wore them.

History
In Reigning Pain in New Orleans, Elijah and Rebekah intervened after learning that Klaus has ordered Diego and the other vampires to attack and kill the werewolves in the Bayou. The vampires returned later that evening to once again attempt to attack the werewolves, so Elijah and Rebekah intervened once more and saved a werewolf named Cary. They then discovered a ring around Cary's neck, which they recognized as having once belonged to their mother, and assumed that she must have given it to her werewolf lover before she turned her children into vampires. This ring marks those werewolves as Klaus' biological father's descendants and Klaus' werewolf kin.

In Moon Over Bourbon Street, Klaus met with Jackson, the Alpha of the Crescent Wolf Pack, during a party thrown by the Mikaelsons, and told him that he wanted the help of the werewolves to protect his daughter, because he believed that the werewolves' family values and clan loyalty would always keep her safe, unlike his fellow vampires, who he believed to be the antithesis of family loyalty. In order to give the wolves the strength to fight a fair battle against the other factions in New Orleans, he revealed the existence of the ring to Jackson and Oliver and explained that believed his mother had given the ring to his father because she had been looking for a way to spare him from having to turn on every full moon. He then informed them that if he is correct in this assumption, finding and replicating the spell would not only protect the bayou werewolves, but, if modified, it would also give them full access to the enhanced powers they gained in their wolf form on the full moon at any time; speed, strength, agility, and their werewolf bite, which is lethal to vampires. Despite Jackson's distrust of vampires and the Original family, his desire for an empowered pack led him to agree to Klaus' deal.

In The Big Uneasy, Klaus asked Oliver to find his descendant, Cary, and bring him to St. Anne's Church for a meeting. Cary, who didn't know Oliver, didn't cooperate with him, forcing Oliver to turn to violent methods to get him to the meeting, which ultimately angered Klaus, who wanted Cary to be treated as his guest and his family. Oliver then argued that his own pack, the Crescent Wolf Pack and Cary's pack, the Northeast Atlantic Clan, have been fighting for generations. Klaus revealed that the reason why he wanted to speak with Cary was to find out what kind of stone their family ring held. However, Cary has never seen the ring with the stone in the setting, so Klaus sent him on a trip to find out. Hayley learned of the deal that Jackson and Oliver made with Klaus for the rings, and though she was resistant at first, she eventually gets on-board at the thought of never being forced to turn again.

In The Battle of New Orleans, Klaus learned from Cary that the moonlight ring spell requires black kyanite stones, and sends Jackson and Oliver to obtain enough of the stones to make rings for the Crescent werewolves. On their way to deliver the stones to Klaus, they managed to walk into a trap while driving on the backroads, as Marcel and Diego, who knew these empowered werewolves would result in the extinction of the vampire species, had enlisted the help of the police to block off the roads so they could be captured. Marcel and Diego then took the wolves to a warehouse to torture them until Jackson made a deal with Marcel to release Oliver in exchange for all the information Klaus had given him about their plan for moonlight rings.

Marcel and Diego ultimately left Jackson at the docks, surrounded by crates of explosives that were rigged to blow when Jackson was ultimately rescued. Klaus and Elijah arrived just in time to find him and rush him to safety before the bombs detonated. Since Marcel destroyed their stones, Klaus was forced to resort to his backup plan, which was to enlist Francesca Correa -- a known mobster who they believed to be human due to her being the representative of the Human Faction into helping him obtain more stones. She delivered them to the compound, accompanied by her brothers, and suggested to Klaus and Elijah that the Original vampires and the humans combine their efforts against Marcel, to which the Mikaelson brothers ultimately agree. When the full moon hit its apex, Genevieve began her spell, which she had created out of three different spells of Esther's, and which she claimed required the blood of a werewolf who doesn't turn on a full moon. She initially suggested Hayley as a pregnant werewolf who didn't turn during the length of her pregnancy, but Klaus, not wanting to put his daughter at risk, offered his own blood instead, as he is a hybrid who can control his own transformation. He assured her that if she had any tricks up her sleeve, he would torture her mercilessly, but Genevieve simply took his blood, dripped it on the stones, and began the spell.

While the spell was in progress, Marcel and his vampires invade the Mikaelson compound in an attempt to stop them from producing the moonlight rings, knowing that if they succeed, it would result in the deaths of all the New Orleans vampires. Klaus chased after Marcel, which left Elijah to fight against the rest of the vampires alone. Francesca and Hayley stayed with Genevieve to protect her while she completed the spell. Once it was finished, Hayley went to take the stones so she could deliver them to the wolves in the Bayou, but Francesca instead stole the stones for herself. Hayley angrily reminded her that a human could never stand a chance against Klaus, but Francesca revealed that she's not actually human before using a knife to kill her bodyguard. Just then, her eyes glow gold, revealing that she was actually a Guerrera werewolf who had waited until she had the means to control her transformation to trigger her curse. Genevieve then used magic to knock Hayley out, both to prevent her from interfering further and because Hayley had been promised to the witches as part of their deal with Francesca, revealing that she and Francesca have been working together the whole time. Francesca passed the moonlight stones out to her brothers, who also killed their own bodyguards to trigger their curses. Since they had the means to control their transformation, Francesca and her brothers used them to bite every vampire who invaded the compound, along with as Elijah, though as an Original vampire, the bites could not kill him. Many of the vampires died in the struggle, and the rest were left weakened and desperately in need of Klaus' blood to cure them.

Genevieve then confronted Klaus in the Quarter and revealed that her spell used Klaus' blood as a power source, which meant that any werewolf who has one of the moonlight stones that Genevieve enchanted with his blood would be drawing on Klaus' strength in order to control their werewolf transformations. This resulted in Klaus becoming incredibly weak, which ultimately made him even more bloodthirsty and feral to try to recoup his diminished strength. It also caused him pain and kept his wounds from healing.

In Rebirth, Elijah reported to Klaus that he had finally located all twelve moonlight rings that Genevieve forged from his brother's blood, leading them to put their plan to take the rings back into motion. Later, after obtaining the twelve rings by attacking and killing the majority of the Guerrera werewolves who possessed the rings, Elijah destroyed them using a combination of the elements water and fire.

In Alive and Kicking, Esther revealed to her newly-"resurrected" sons Kol (possessing Kaleb Westphall) and Finn (possessing Vincent Griffith) that she had the French Quarter witches making new moonlight rings around the clock, with the intention of using them to buy the New Orleans werewolves' allegiance and using the alliance between the witches and the werewolves to go after Klaus and Elijah.

In Live and Let Die, the werewolves who had been empowered by Esther's moonlight rings, including Aiden and Oliver (who was working as a double agent for Hayley), stormed Marcel's apartment and warned them that they had decided the vampires needed to leave New Orleans completely or risk death at their hands the next night. However, this threat was put on hold when it became clear that Finn planned to force over a dozen untriggered teenage werewolves from their packs to activate their curses by killing Finn's human opponents; instead, Aiden and Oliver came to Hayley, Elijah, and Marcel for help in saving the teenagers from this fate.

In Keepers of the House, Freya gave a moonlight ring to Keelin, but placed an additional spell on it that has allowed her to control the werewolf and prevented her from taking off the ring. However, Freya later decided to free Keelin by lifting her coercive spell on the ring after they start to bond.

Powers And Abilites
The rings give werewolves the full power of their wolf form while also protecting them from being forced to turn on the full moon. This gives them access to a range of abilities at all times that they would ordinarily only have in their wolf form during a full moon.
 * Super Strength - The werewolves who have moonlight stones gain the full strength from their werewolf form, making them strong enough to overpower vampires without having to transform.
 * Super Speed - They also gain the full speed from their werewolf form, without having to transform. With this, they were seen giving chase to the vampires fleeing after Francesca distributed the stones for her brethren.
 * Werewolf Bite - They are able to grow fangs that produce the same lethal venom that they gain in werewolf form without a full transformation. In doing so, they gain an advantage against even the oldest and most powerful vampires, who will become weakened and eventually die from the venom. Those these bites are not fatal to an Original vampire, the venom is still toxic enough to weaken them and cause them to suffer hallucinations while it leaves their system.
 * Super Agility - They will possess the full super-human agility from their werewolf form. While in human form, they will be able to jump at extreme height and also climb, flip and run extremely fast at will without difficulty or exhaustion.
 * Enhanced Healing Factor - They will acquire a slightly stronger regenerative ability.
 * Lycanthrope Enhancement - They will have the ability to make their eyes glow at will, which at the same time will not only increase their strength but their speed as well.
 * Transformation Control - They are shielded from the effects of the full moon and do not need to transform, controlling the beast inside them and presumably allowing them to transform at will.

Weaknesses
The rings do not make werewolves immortal and as such, they retain their normal weaknesses, such as wolfsbane. The rings themselves seem to have a weakness as well -- the elements of water and fire can destroy them. A werewolf with these rings doesn't seem to have their powers passively and likely needs to call upon them, perhaps explaining how they were easily killed by vampires and a newly-turned hybrid.

Only werewolves who have triggered their curse can access the power of the moonlight amulets, since Finn had to spearhead a group of werewolf teenagers into a midnight massacre in an attempt to activate their powers before bestowing them the rings.

Trivia

 * The rings grants werewolves the ability of Transformation Control that would otherwise only be naturally available to hybrids.
 * Because Genevieve's spell required the blood of a werewolf who doesn't turn during the full moon, this possibly means there are/were werewolves who could control the change without transforming. However, based on Genevieve's hypothesis, a pregnant werewolf can also fulfill this role and (presumably) an untriggered werewolf as well, since both these types of werewolves don't transform during the full moon.
 * Genevieve's spell also required that the stones be enchanted during a full moon, when the moon is at its apex, just as the daylight amulets for vampires have to be bathed in sunlight in order to protect its wearers from the sun.
 * The spell Esther used to make the rings do not use the blood of a werewolf who can control the shift, hinting at an alternative method to create the rings. They also do not seem to need to be made on the full moon, as the French Quarter witches were making them in the middle of the day.
 * Their vampire counterparts are the Daylight Amulets.
 * Moonlight rings do more for their wearers than their counterparts do for vampires, as the daylight rings only grant protection from the sun, while the moonlight rings not only protect their wearers from the full moon but also grant the wolves the ability to call upon their werewolf form's power.
 * However, it is suggested that the very first moonlight ring made by Esther for Ansel in the late 10th/early 11th century only prevented its wearer from being forced to turn. It wasn't until Genevieve modified the spell to include the ability to control their transformation by accessing the power of their wolf forms that the moonlight rings could also be used to grant enhanced powers to the wearer.
 * Esther is possibly the first one to make a moonlight ring, as she was deeply involved with a werewolf. There may have been more created since then, but it's likely that few exist since witches tend not to associate with werewolves, and vice versa.
 * Niklaus' father, Ansel, is the first confirmed wearer of a moonlight ring, though his descendant, Cary, wore the ring without a stone around his neck.
 * From what was revealed in The Battle of New Orleans, the enchanted black kyanite stones used to create moonlight rings and amulets do not have to be made for a specific werewolf. Instead, a werewolf only needs to have the stone somewhere on his or her person in order to be protected from a full moon and able to control their transformation. Francesca and her brothers, for example, only had the spelled stones (which had not yet been attached to jewelry) in their pockets during the full moon.
 * The moonlight rings won't work unless they have the correct stone. In The Big Uneasy, Klaus enlisted Cary's help to find out what stone it was. In The Battle of New Orleans, it was revealed that the stone needed was black kyanite.
 * It's the spell for the moonlight stones that Genevieve created was adapted from three separate spells in Esther's Grimoire.
 * If a werewolf uses one of the stones enchanted by Genevieve, which used Klaus' blood, the werewolf will draw on Klaus' strength for the power to control their transformation. As a result, Klaus will feel extremely weak and will be in a great deal of pain during the full moons. These symptoms vary in severity depending on how many werewolves are using the stones that were spelled with his blood.
 * Elijah and Klaus planned to gift a ring to Hayley.
 * It is unknown if a hybrid in human form is stronger than a werewolf with a moonlight ring. Though it is unlikely, since the rings give a werewolf the full power of their wolf form without actually transforming, which gives them advantages i.e they can control their actions. Hybrids, on the other hand, have only vampire strength which depends on age, diet and only human werewolf strength which is inferior to even a newborn vampire's strength, however, hybrids have the capacity to transform at their choosing and thus gain access to their full powers, in which case they would indeed be far more powerful than a moonlight wearing werewolf, as they will have vampire strength to go with their werewolf form though they will lack control. Hayley, a newly-transitioned hybrid single-handedly killed multiple Guerrera werewolves, many of whom were wearing moonlight rings. She also easily killed Francesca, who was also wearing a ring.
 * According to Michael Narducci, moonlight rings probably don't work on hybrids as they can already control their transformation. [1]
 * Due to Hayley not offering the rings to Kayla, it's possible that they were destroyed.