Devouring The Dark River
The map Fruity got from his past self showed that he would have to cross the Dark River before reaching Illusion Mountain. The river was a mystery, even to Yuying, who had no concrete description of it.
All she said was that the river had a guardian that lived within its depths, and from what she knew 50 years ago, it had been a Beginner Tyrant (Tier 8) back then.
And 50 years was a long time, so she estimated it had likely become a Deimos (Tier 9) by now. Aside from that, there were river monsters that attacked using the water element.
Fruity, of course, was prepared for a huge fight, knowing that the guardian wouldn't attack unless provoked, according to Yuying. He planned to cross the bridge quickly since flying wasn't an option there.