It was already a week since Sebastian's Leviathon body departed.
Sebastian departed from the Raging Fire military base with 1 main motivation pushing him on which was the urge to join the war by all means as it would serve as the perfect platform for him to grow stronger faster.
In the end, the ultimate reason why Sebastian decided to join the war was to level up faster, increase his origin law comprehension faster, and get to the peak of the purple tier realm as fast as possible.
Fulfilling these goals was the only way for him to draw closer to his real goal of becoming the Ocean Master, and he was determined to fulfill them.
Because the more he evolved, the harder it was to level up, Sebastian was under a lot more pressure to level up faster.
The main advantage that he had was his strength.