During the battle, Sylas had focused on dodging and made use of his telekinesis to keep the water turbulence from getting too great. After silently killing their guardians, there was no one to stand in his way, and there was also no one who had even noticed what was going on here.
Kargan had focused all his elites at the front gate, and that gave Sylas free rein.
Before he had come, he used <Astral Will Fission>. Although he didn't dare to get too close to most here, he had a good understanding of the layout, and he knew where to find his targets.
He slipped onto the dock and killed the two guards in quick succession. They were silently pulled into his Madness Key, and no one was the wiser.
After this, he had an easy stroll into the village.
He moved from home to home, killing those he came across swiftly and silently. Even when there was a slight scuffle, the noise was within reasonable enough limits that it didn't alert many at all.