The Eye of Elena is an ancient necklace said to grant magical powers of protection to its wearer.
In actuality, it is the Lock - an object capable of binding the Wyrdkeys to the broken Wyrdgate, and ensuring the Wyrdgate will never be opened again.
The Lock is a warped metal composed of two overlapping circles inside a greater circle. The heart of the amulet is in the shape of an eye - with a blue gemstone in the middle.
It is revealed that the blue stone is actually a witch mirror, capable of absorbing any amount of power. Rhiannon Crochan created the basis of the Lock, and helped Mala Fire-Bringer cast her essence into the blue witch mirror, giving up her mortal body and imbuing the Lock with her power.
Manon Blackbeak reveals that the Amulet should in fact not be called the Eye of Elena, as it is the Eye of the Goddess, with the Three-Faced Godesses' three forms: the exterior circle representing the Crone, the upper interior circle representing the Maiden, and the lower interior circle representing Mother. The eye in the middle is known as the Eye of the Goddess, in which lies the heart of the Darkness, of the Three-Faced Goddess.
The amulet first appears when Elena Galathynius Havilliard, first owner of the Eye, gives it to Celaena on the tombs for her protection. It is then, when Celaena is sent to Wendlyn, given to Chaol. In the final book, Queen of Shadows, it is again given to Chaol in the final show down with the King of Adarlan, deflecting against the Wyrd Keys power and saving his life.
When the second Valg wars erupted, led by Erawan, the Gods and Goddesses made a deal with Mala and Brannon. They would be sent home through the broken wyrdgate, and then the three wyrdkeys would be sealed shut in the gate - forever rendering the gate unable to open for armies, and to destroy worlds. The returning offer that stood between them was that they would take the Dark King Erawan, including his armies, and tear him apart in their home world. The cost of the Lock though was enormous. Mala was required to give up her mortal body, and her life with her mate and children.
Rhiannon Crochan created the basis of the Lock: the symbol of the Three-Faced Goddess in an amulet - a warped metal with two overlapping circles, with a rendering of the godess' "Heart of the Darkness" in the middle. Rhiannon crafted a witch mirror in the middle eye, which was capable of absorbing immense power. The amulet then, had been called the Eye of the Goddess.
With the help of Rhiannon Crochan, Mala cast her entire mortal being, her full Fire-Bringer gifts, and her whole essence into a small blue witch-mirror in the Eye of the Goddess. Though she had given up her life to forge the Lock, she was still able to live on as a goddess.
Mis-use of the Lock
The Lock was only to be used once, it's power cabable of sealing anything the bearer of the amulet wished.
Approximately ten years after Mala gave up her life for the Lock, Erawan's armies had nearly gotten full control of the continent. Brannon's fire magic was dimming, and he was said to be only decades from the Fading.
In a desperate attempt to seal Erawan and stop the spread of his darkness and pain across the world, Elena Galathynius stole the the Lock from her father, and used it to seal Erawan in a tomb in Morath. She did not know the Lock's purpose, or that her father bore the Wyrdkeys. Upon ruining the bargain between the Gods and Brannon, the Gods expressed their rage, however Mala Fire-Bringer paused in her wrath, as if trying to remember Elena.
The Gods and Goddesses proposed that a new Lock would need to be forged, by a scion of Mala blessed with her same Fire-Bringer gifts, who would have to give up their life to seal the Wyrdkeys into the Wyrdgate. They informed her that their original bargain still held: they would destroy Erawan in their homeworld. And so Elena was tasked to lead Aelin to find the Wyrdkeys, forge the Lock, and give up her life to seal the gate shut. And when Elena brought Aelin back to life after she died in the Florine river, she was given an additional punishment - she would never be able to access the Afterlife, she would merely disappear after Aelin completed her task, and never see Gavin or her children again.
The Amulet is triggered by courage of the heart, deflecting against powerful Wryd magic, and warning against danger.