Hidden Duality Help

[Tranth]

I picked this one up, since it was one of the few 5 star quests, and have been really impressed with the complexity of the first 2 dungeons, but I'm stymied now. Level 3 is in apparently in a desert, but I haven't even seen anything that looks like a desert, and I've seen as much of the overworld as I can. I feel like I'm missing something really obvious, here. Any pointers?

[Cloral]

There are 2 overworlds. Try looking for a portal.

[DarkFlameWolf]

how's this for being specific: there is a portal to the dark world just ONE SCREEN SOUTH of the level 2 entrance. Try using various methods to get to that screen!