Eclipse (エクリプス Ekuripusu) is a Magic Item created and used by combining Celestial Spirit Magic and a certain Magic from the "Book of Zeref". It takes the form of a giant gate that enables people to travel through time when the sun and moon intersect. 
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.
The gate requires a large amount of Magic to use and will absorb any Magic used within its immediate vicinity. It can store levels of Magic rivalling those of Etherion. The twelve Zodiac Keys are required to unlock 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 Eternano that resembles Zeref's Magic power lingering around them. This particular phenomenon would prompted Mages with sensor ability 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 a "door" that simply connects two time periods. Additionally, Lucy Heartfilia states that the event of a full moon could disrupt the Magic power contained in it, resulting in the gate running rampart.
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".
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 Datong 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.