Ephira was a former hamadryad who became twisted and darkened into a spectral being of fear.
Ephira was once one of the many hamadryads in Fablehaven, and was said to be flirtatious and bubbly. She fell in love with Marshal Burgess, Patton Burgess's uncle, who was the caretaker then. The love was mutual, so she renounced the trees and married him. Their courtship was said to be brief and passionate. She quickly became Patton's favorite relative. Patton once said that it was through her guidance that he became fairystruck. Unfortunately, the relationship between Ephira and Marshal was short-lived due to him quickly returning to his philandering ways. It unraveled within months, and Ephira was crushed, having given up immortality to be with him only to be betrayed. The anguish and betrayal poisoned her reason. Ephira abandoned Marshal and disappeared, becoming obsessed with regaining her status as a hamadryad. She visited many for help, including Muriel and the swamp hag, who directed her to Kurisock the demon. Kurisock told her to cut down her tree with her own hands, burn it, and bring the last seed to him. Ephira obeyed him, and Kurisock bound himself to the seed and reforged the bond between Ephira and the tree. Unfortunately Ephira did not turn back into a Hamadryad, instead becoming something else. She was dark and spectral, tainted by demonic power, a negative of her former self. Merging with Kurisock only magnified her vengeful feelings. She then returned to the manor, destroying Marshall and the others that lived there. Patton only barely got away, managing to take the key pages to the treaty from the register before he fled. Since then, she has inhabited the manor alone, only regularly leaving on Festival Nights.
Ephira is seen twice in the first book. Once, she is outside Kendra and Seth's attic window on Midsummer's Eve. When Seth opens the window, thinking to save a baby that is a monster in disguise, he lets in several monstrosities. Fighting her way to the window, Kendra slams it shut, preventing Ephira from entering the house. Despite this, her eyes haunt Kendra for several days after, having held her spell-bound and draining away all hope.
Later, Ephira is seen battling alongside Muriel after she has released Bahumat. She stays for a short time, draining the vitality from two giant fairies from Kendra's army, but leaves after several others slice off a large number of her tendrils and the tide of battle turns against the dark creatures. The fairies do not pursue her.
After Seth escaped from Olloch the Glutton, he found the Old Manor, which Ephira inhabited. She discovered his presence as he was preparing to leave and attacked. As Seth had not had any milk that day, he did not recognize her, appearing to be a whirling storm of dust and wind. He managed to escape into the woods, and she did not pursue him. When the Sphinx allowed Navarog to escape, Navarog took the Revenant's nail and gave it to Ephira and Kurisock. Ephira put the nail in her new tree, increasing her own power as well as Kurisock's and making the darkness that he had used to transform her highly contagious.
With Kurisock and the nail, Ephira began the Shadow Plague, which infected most of Fablehaven's inhabitants, turning them into dark, twisted versions of themselves. While the mythic creatures infected joined her side, the humans infected remained self-possessed, although reduced to shadows that only a Shadow Charmer can hear. Those creatures and people not infected were forced to flee to the naiad's pond with the Fairy Queen's island shrine, which was protected from any being infected by the Shadow Plague. Once the Sorensons were there, Seth hinted that whichever being lived in the old mansion might be the source of the Plague, and Lena, who was once again a naiad in the pond, told them that Patton had hidden the Chronometer in the same mansion. Seth goes with his grandparents, Dale, and Warren to raid the manor and retrieve the Chronometer, where they discover that the dust devil Seth saw on his previous visit was actually Ephira. She is much more powerful than before, radiating paralyzing fear despite none of the humans having consumed milk or walrus butter (which should have made them much more resistant to the fear effect and less likely to be detected by her). She is also immune the the crackling dust that normally repels creatures of darkness. Seth notes that cords of darkness stretch from her out into the rest of the preserve, invisible to the others, and it is deduced that these are her connections to the victims of the plague. Dale, Warren, and the Sorensons are all captured and turned by Ephira. Seth manages to evade Ephira long enough to find the Chronometer, and accidentally brings Patton Burgess (Lena's deceased mortal husband) forward in time several decades. Patton's appearance surprises Ephira, giving him and Seth a chance to escape out a window. According to Patton, Ephira detests sunlight and was never very fast anyway, so she will not follow them.
Seth and Patton return to the pond with Hugo and the Chronometer. He then woos Lena away from the naiads again and explains Ephira's past. A short time later, Kendra receives a token from the Fairy Queen, a small pebble infused with light and magic. Touching it to the nail will destroy the nail, Kurisock, and Ephira. However, the person who touches the two talismans together will die instantly. The next day, with an army of satyrs, hamadryads, fairies, dryads, and many others sets out to battle their way to Kurisock's tar pit and Ephira's tree. Upon entering Kurisock's domain, they encounter the bulk of the infected creatures. Kendra quickly discovers her touch, when complemented by the power of the pebble, is able to overcome and cure the Shadow Plague. As the attackers come in sight of the tar pit, Ephira arrives. She quickly begins to turn many of the uninfected creatures, almost as fast as Kendra can cure them. Ephira attempts to transform Broadhoof, but as Kendra is riding on his back, the warring powers of light and dark kill him instead. Ephira is unable to touch those the pebble with her fabric tendrils and attempts to paralyze the mortals with fear instead. However, after a short but costly battle, Lena takes the pebble and connects it to the nail. Kurisock is slain and the tree loses its magic, rotting to the core. Ephira is returned to mortal form, and for a brief moment she is simply a frightened woman in black rages. Then she ages until her body crumbles into dust. Their deaths end the threat of the Shadow Plague.
She appears as a beautiful woman swathed in writhing black garments which she can control with her mind. She has black hair that floats around her translucent face. Her eyes are empty, and she radiates magical fear that Seth can resist because he is a Shadow Charmer.