The mission Rean looked at was quite specific. It required them to help look for a certain someone in there. Anyone who found any clues of the person inside would be rewarded. It didn't seem anything out of the ordinary. After all, there was no lack of people who entered a forest teeming with Demon Beasts just to never appear again.
One of the things that caught Rean's attention was that this request talked about a Middle Stage Core and Soul Fusion Realm cultivator disappearing in there. Even if the guy or girl lost in a battle, they would still be able to run at such a level. Especially since Stage Four Demon Beasts haven't developed real intelligence with sentience.
Of course, just that wouldn't be enough to catch Rean's attention. The part that interested him was that the guy disappeared around the region Rean and Roan had to adventure into. Since they were already going there, they might as well keep an eye open.