Eclipse (エクリプス Ekuripusu) was a Magic Item created and used by combining Celestial Spirit Magic and a certain Magic from the Books of Zeref. It takes the form of a giant gate that enables people to travel through time when the sun and moon intersect in a Solar or Lunar Eclipse.
The Gate has a large door with an elaborate design. At the center of the door is a sun with a darkened moon below it. The lower section of the door from the sun past the moon is dark blue with golden lines, the upper section is green and split into twelve segments. Around the sun are several circles, the innermost one connecting the moon and twelve holes, one in each segment. A set of steps leads up to the door and two pillars stand on either side of the door with an arch connecting them. Atop the arch is an emblem consisting of an eclipsed sun. On either side of the Gate is an additional support, fashioned in the shape of a leg. It is made from a super strong, Magic-resistant metal known as Maginium.
The Gate requires a large amount of Magic to use and will absorb any Magic used within its immediate vicinity, however, the Magic-draining feature of the Gate ceases to function when it is open. It can store levels of Magic rivaling those of Etherion. The twelve Zodiac Keys are required to unlock the Gate, as well as close it. If a key is missing, the life force of a Celestial Spirit Mage can be used as a substitute.
Due to the origin of this particular Magic is related to the Black Mage Zeref, anyone who uses the Eclipse Gate would have residual Ethernano that resembles Zeref's Magic Power lingering around them. This particular phenomenon prompted Mages with sensory abilities to mistakingly identify the users as Zeref himself.
According to Crux's analysis, the Gate's purported feature as a cannon is non-existent, as it is simply a "door" that connects two time periods. Additionally, Lucy Heartfilia states that the presence of a full moon disrupts the Magic Power contained within, resulting in the Gate running rampant.
The Gate's original purpose was for its creator, Zeref, to go back in time to stop his younger brother from dying, though he later abandoned this idea. Instead, Eclipse was used as part of a collaborative scheme between Zeref, the Celestial Spirit Mage Anna Heartfilia and the surviving pro-cooperation Dragons to defeat Acnologia.
In the present, the Gate is the subject for a plan known as the Eclipse Project (エクリプス計画 Ekuripusu Keikaku). By using Magic siphoned from the participants in the Grand Magic Games, the twelve keys and a Celestial Spirit Mage, the Gate is set to open on July 7, allowing some individuals to travel back 400 years in time where they would kill Zeref before he could attain immortality.
The project was a cover for another plan, intended to avert the destruction of Fiore as told to Hisui E. Fiore by an individual who claimed to be from the future. This second plan involves making use of the large amount of Magic stored within the Gate over the past seven years. The second plan is known as the "Eclipse Cannon". This is actually a deception as the Gate can't operate as a cannon.
Due to not being able to recover their wounds in the current time due to the lack of Ethernano, the Celestial Spirit Mage Anna helped Zeref with his plan to send five Dragon Slayer children (who were used to house the tattered souls of their foster parents via the Dragon Soul Technique) to a point in the future in where they can live within an era with a surplus amount of Ethernano while also preparing to defeat Acnologia. The projected time when the Ethernano would have been dense enough for the Dragons to recover was passed down through Anna's clan, and her descendant Layla Heartfilia opened the Gate on July 7, X777, with Anna as well passing through it but could not reveal her motive of finding a way to defeat Acnologia was at the time. Lacking Aquarius' key, Layla substituted her own life force in its place, causing her to fall ill with Magic Deficiency Disease.
Many years later, following the conclusion of the fourth day of the X791 Grand Magic Games, Arcadios and Yukino bring Lucy, Natsu and their friends before the gate while attempting to persuade Lucy to help with the Eclipse Project. When Darton orders his men arrest Arcadios, Yukino and Lucy; Natsu attempts to stop them with his Magic, but the Eclipse Gate completely drains it, leaving him unconscious. Afterwards, Hisui uses the confiscated Celestial Spirit keys to open Eclipse's locks.
On the sixth day of the Games, with the final event having ended as her informant from the future said it would, Hisui initiated the plan and the Gate was brought to the surface. The moment the Gate is opened, a flight of Dragons emerges and starts wreaking havoc atop Mercurius. The Gate, however, is swiftly closed by both Lucy and Yukino, who, together, utilize the connection between the Gate and the 12 Ecliptic Zodiac Keys.
Later, following a note in the future Lucy's diary, Lucy and Yukino attempt to destroy the Gate to prevent Rogue from using it in the future. Their attacks are ineffective, but Natsu's last attack against Future Rogue and Motherglare results in the Gate's destruction. The resulting time paradox forcibly returns the time travelers to their original time periods, though the effects and memories of events caused by them remain, while the five Dragon Slayers and Anna were unaffected by its' destruction.