After he killed the first, Luke had an easy time defeating the other, giant fish. He just had to keep his katana charged for a while and then unleash the attack. Still, he only found those creatures after a few hundred or so kilometers. Luke assumed that it was because they lived in territories. They couldn't live together since food was scarce and they were too big.
Regardless, when he killed the tenth of those fish, something happened. Luke felt the hairs on his back standing, and in the next moment, a beam of energy was fired at him. It was the size of the whole, and its speed was surreal… Luke didn't even have time to dodge. He only escaped because he ran away to his dimension. Using his skill like that wasn't good, but it couldn't have been helped…
