First, you must get the lizard tribe good ending. After that, confront Rhot of his little nighttime activity of him spying on you on your sleep (you'll get this scene by sleeping in the lizard tribe at night around 20:00 - 23:00 anyday, where you get the feeling that someone's watching you in your sleep. Which is Rhot).
Lastly, just wonder around in the corrupted lake, and there's a chance you'll run into a trap made by Xer. Once you got caught, you'll eventually encounter him.