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 rivalling those of Etherion. The twelve Zodiac Keys are required to unlock the Gate, as well as close it.
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 identify the users as Zeref himself.
According to Crux's analysis, the Gate's 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 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 7th, 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.
After the 4th day of the 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 gateway completely drains it, leaving him unconscious. Afterwards, Hisui uses the confiscated Celestial Spirit keys to open Eclipse's locks.
On the 6th 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. It was then destroyed so as to send both the Future Rogue as well as the Dragons back to their respective times.
On the 6th 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. On the morning of the 7th day, the gate is fully unlocked and then opened, with the intention of preparing to unleash the stored Magic against the dragons, causing an eclipse to occur.
However the gate actually has no such function and the presence of a full moon causes the gates to be locked on to a set time period - the time when the Dragons lived. Shortly after the gate is opened, Dragons begin emerging from the gate and proceed to wreak havoc in Mercurius. As more Dragons appear, Lucy attempts to close the gate using the lever next to it and her Magic, but without success until Yukino arrives. Combining their Magic, the two open all 12 of the Zodiac Gates and the Celestial Spirits manage to close the gate, preventing any more Dragons from coming through.
On reading the note in Future Lucy's diary that destroying Eclipse would prevent travel back in time through it from the future and cause those who did to cease to exist in the present, meaning Rogue couldn't travel back to cause the events that led to the Dragons coming through, Lucy and Yukino use their Celestial Spirits to attack the gate, but are unable to damage it. However as Natsu's fight with Future Rogue concludes, Natsu's final attack sends Motherglare, Rogue and himself hurtling towards Eclipse, and the resulting impact destroys the gate.
The destruction of the gate means that Rogue cannot use Eclipse to travel back from the future and cause the events that have occurred. As a result of this time paradox, the Dragons and time travelers are forcibly returned to their original time periods, but due to time being heavily distorted, the other effects and memories of the events caused by them remain.