User blog comment:SATOSHISUMI/Mystery Woman/@comment-80.213.205.87-20140105160547/@comment-7418318-20140105163313

SPOILER ALERT

The original intent was to open a gate to 400 years in the past to kill Zeref before he became too powerful. Later someone convinced princess Hisui to change this project to a cannon intended to kill an invasion force of 10.000 Dragons.

EVEN MORE SPOILER

As it turns out, opening the Eclipse Gate during the lunar eclipse is making it go out of control, which was the exact cause of the Dragon invasion in the other timeline. But in the "regular" timeline only seven made it through before the gate was closed by Lucy and Yukino.