They had killed the Hydra-ling, yet they were stuck in Lerna. They didn't get any rewards either, which only meant one thing. The quest wasn't over.
"...of course this would happen," Yeji mumbled. "Look at the quest prompt."
[Objective:]
[>> Survive for two days on the lake without being killed.]
[>> Alternatively, kill the Hydra to clear the trial.]
"We haven't completed either of the tasks," Lucifer mumbled.
The quest stated they had to kill the Hydra, not a Hydra-ling. The only problem was that a Hydra-ling wiped out more than half of their initial force. Now they find out they were fighting the wrong monster all this time?
I even had to get naked because of it...