The Myth Realm Planetary Dimension.
After staying in this remote part of the universe for over a millennium already, Oceania finally moved, intending to return home.
For this planet-sized war fortress weapon to move, all that Sebastian had to do was to send one of his living doppelgangers to the brain of the weapon.
There, he supplied constant mana that became the fuel that powered the massive engine called Oceania. The complicated machinery and magic construct geared to life as Oceania became a moving planet.
Under Sebastian's control, Oceania set a course for the myth realm.
The journey was going to be a fast but long one and was why Sebastian made extensive plans for it. Taking the speed of the invaders as their reference point, Sebastian estimated this journey to take approximately a year.
To him who only spent a year and a few months in Oceania, this was a long time but Sebastian could do nothing about it so he could only cope with it.