I have a theory that Esther is actually a Kitsune. Since we haven't seen that much of Esther my theory may not seem too solid, but I do have good reasons for thinking this. I have done my research on Kitsune, so I know what Kitsune are capable of, what kind of powers they usually possess, and how they are usually 'portrayed. If you'd like me to provide my sources I will. Tell me your thoughts. I'll also be adding more theories as I come up with them, and add more to old theories if I feel the need to.
1) Kitsune Star ball/Jewel and Esther's necklace: When Kitsune possess someone or when they shapeshift, they put a portion of their power inside their Star Jewel. Esther's necklace possesses a portion of her power. Perhaps Esther was possessing Vicki (hence the darkness Anna felt) It's true that Kitsune don't normally possess ghost, but that doesn't mean they're unable to. It should also be noted that Kitsune possess the power to use fire and lightning (Kitsune-bi) and Esther's necklace burned Elena (fire) and zapped Bonnie (lightning) when they touched it. Let's not forget when Rebekah touched the necklace and Ayanna burned her, like she was protecting it. Or perhaps it was the necklace itself that burned Rebekah and Ayanna allowed Rebekah to think it was her that burned her to protect Esther's secret. Esther's name also means "Star" go figure. Esther's necklace only seems to give off magical powers whenever Esther is around and active. It showed powers in Ordinary people, and it showed powers when Vicki showed up. As I said before, Esther could've been possessing Vicki or she could've shapeshifted into Vicki, and whenever Esther does either of those two, a portion of her power goes to her necklace (Star Jewel) In the novels Damon uses Misao's Star ball to open the gate to the dark demension, and in the series Esther's necklace is used (temporarily destroyed) to send away the ghost and close the door to the other side.
2) Kitsune and Magic: Kitsune are capable of studying and using any normal field of magic, and this includes witchcraft. However, the drawback they possess is that in able to use magic, that isn't natural to them like Fox Magic is, then they need to draw from another source for power (like Nature or Witches) because if they draw from themselves, they will become weak, and their power will not replenish unless they feed on essence. Esther has mentioned that she drew power from the Bennett witches, both in the past, and present. She also tried to get Ayanna to help her turn her children into vampires, but she refused, forcing Esther to do it on her own (which probably weakened her) It is unknown if Esther got help to put the hybrid curse on Klaus, or to make the rings, but if she didn't then that would've further weakened her, which would explain how Klaus killed her, she was too weakened to defend herself. Kitsune are not natural witches, which would explain why Esther doesn't like doing spells on her own, and why she draws power from the bennetts. It was mentioned in Murder of One by Klaus that Bonnie's energy helped Esther link them together, which means that Esther drew power from her.
3) Kitsune do not age: Just look at Esther, she looks the same age as Elijah, Finn and Klaus. At the Mikaelson ball, she looked as young as Rebekah and Kol. Campare her to Mikael and you'll see a huge difference. Kitsune also tend to feed off of human essence (usually through sexual contact) This can cause the victim to slowly wither. That could explain why Mikael looks a lot older than Esther, and that might also explain Esther's reason for having an affair.
4) Kitsune have a fear and hatred of dogs: If this refers to wolves as well, then that would explain why Esther was so indifferent towards Klaus.
5) Kitsune follow their own code of ethics: If someone offends what a Kitsune believes is correct, they can become evil, malicious, and disruptive. If one tried to stop Esther from doing what hse felt was right, this would count as an offense. This would explain the Darkness that Anna felt whenever she tried to stop Vicki and why Esther opened the door to the other side. If someone behaves according to their ethics, then they become polite, kind and helpful. This would explain why Esther was so kind and polite to Elena even after she tried to kill her before.
6) Kitsune and Fire: Fire is a Kitsune specialty, they can create fire and control fire. This doesn't really prove anything, but it would explain why Esther was able to burn the sage so well when Bonnie couldn't. Bonnie kept it burning, but she couldn't control the fire properly. Or it could be simply because Esther is a much more powerful witch, but even when Esther put the sage down to talk to Elena and then give her speech to the crowd, the sage was still burning that whole time. Also after Esther linked her children, she summoned fire to burn the paper. This could be what sealed the spell. Again, this doesn't prove shit, because witches can summon fire to, but 'it should be noted that Esther is able to summon fire a lot faster than other witches. Esther also seems to use the element fire a lot. Burning the grimroes, trying to kill Elena, sealing the spell, burning the White Oak (assuming she is the one who summoned the fire to do so) ect. She can also summon it, and when she does it doesn't look like she's even trying. She doesn't make any facial expressions and she doesn't move her hands. Kitsune are also immune to fire, and if Esther's immune then perhaps her necklace is as well.
7) Esther's children: Children of a Kitsune and a human don't inherit the Kitsune abilities. However, they do possess some powers that set them apart from normal people such as longevity, and sharp senses. This would explain Rebekah's "borderline supernatural" abilities.
8) Kitsune are Tricksters: This should be obvious. Esther tricked Klaus into thinking that killing Elena would allow him to break the curse and sire hybrids. It certainly allowed him to break the curse, but he would still be the only one of his kind. She was also able to trick her entire family in recent episodes.
9) Kitsune are naturally mercurial: Obviously so is Esther. The Esther of the present is completely different from the Esther we saw in Ordinary People. In fact if I didn't know any better I'd say they were different actresses. Earlier in season 3 she was hostile towards anyone that got in the way of her plan to kill Elena, and now during Dangerous Liasions and All My Children she was kind and polite to them. She also tends to act differently when around certain people.
10) Esther didn't become a vampire: Now why would Esther make it so her family could live forever, but she couldn't. If she was willing to betray nature to save her family, you'd think she'd want to join her family. Well, if she is infact a Kitsune then the answer to this is obvious. She didn't become a vampire because she couldn't become a vampire. Kitsune are fox spirits, not mortals, so if she died, it would drive her back into the spirit realm (other side)
11) Witch powers pass from mother to daughter or father to son: Rebekah however, never mentioned or hinted at having powers. Also Kitsune have the ability to use mind control to make people overlook what the Kitsune wants them to overlook. Esther could've easily done that to Rebekah.
12) Abby screamed when she saw Esther come out of the coffin: Now if I saw Esther as she is now, I wouldn't scream. Would you? So perhaps she was in her fox form when she came out then used her mind control powers to put Abby and Bonnie to sleep and make them forget what they saw. You'd think that Abby and Bonnie would question Esther as to why she knocked them out, but Esther could've used her mind control to make them think they just past out from overuse of magic. Also, Esther was physically strong enough to bust open the coffin and jump right out without Abby seeing her. Now if Esther was just a witch I find it hard to believe she would be that strong and that fast. Also notice how the candles flare up.
13) Kitsune can create Illusions: To the Kitsune and all others, their illusions are reality as long as the illusion exist. Esther's necklace being destroyed in the fire by Bonnie and her grandmother could've been one of her illusions.
14) Celestial Kitsune and Inari: From what I've read from numerous sources Celestial Kitsune are the most common, and they are the ones who follow Inari. Jewels are also a symbol of Inari. Celestial Kitsune tend to feed off of magic and sorcerers which would fit perfectly with Esther. Assuming Esther is a kitsune, does that mean she follows Inari? maybe. Could Inari actually be the one who wants Esther to kill her children? maybe. Celestial Kitsune also believe they must gaurd all under their domain. This also fits well with Esther as she was willing to betray nature to protect her family, and she was also described as "a lioness protecting her cubs" Also, take the time to compare the design of Esther's necklace to some Celestial symbols, the celestial moon symbol in particular is quite similar.
15) Kitsune are able to create exact duplicates of a specific person: Whether this means shapeshifting, or using magic to create someone that looks identical, or both is beyond me. Perhaps this is how the Petrova Dobbleganger was created. Esther used her Kitsune magic to make it so an exact duplicate of Tatia would come into existence every 500 years.
16) A Kitsune in human form is usually a young attractive women / Privacy: In my opinion Esther falls into this category. Also, Esther's facial expressions sort of ressemble those of a fox. Or perhaps I'm just seeing that because I want it to be true, IDK. According to Rebekah, Esther fancies privacy. If she's a Kitsune, that would make perfect sense.