It had been more than a year since they found the gorge separating the Castle area from the plains. The bridge was already finished, and it even had a lift function to break off the way in an emergency. Rather than crossing the plains and conquering it, however, they preferred to sail through the river and tried to see if it really connected to the sea.
For that, Mortix had been building a ship for the past year.
And no--of course they couldn't just use an already available ship that wasn't meant to withstand the Deathzone's harsh condition. They had to design it from scratch to maximize protection and resilience, using the data about the fragments and the shards to create the best materials.