Boys, this dungeon has been searched from top to bottom, and my enemy has fled to the Void (By that, I mean the open space behind the walls). I need your help. How can I make my way into that place? This quest is frustrating me like no other.

Hmm... I do wonder if it would even be possible to find him out there. But, it's worth a shot.

Do you have levitation? You need that for voidwalking. Stand at the top of a stairway. Levitate as high as you can go. If you levitate forward, you'll notice that the slope forces you down, but if you duck, you'll notice you can move further forward without hitting your head; do so until you hit your head. Now, levitate up even further... and out!

You can get back into realspace in exactly the same way: levitate up through the stairs.

There's also a harder method I usually go with (not sure why). There are certain doorways near closed off quest areas (usually upper entrances into rooms with some loot, a ramp downward, and a closed trapdoor hatch at the bottom of the ramp) where the doorway is fairly thick. If you climb the doorway to the angle up above and levitate and duck while you get up there, you can stand up and put most of your body into the void. Working back and forth a little you can get on top of the room. Then you can levitate down to the closed hatch right near you, and reach through the wall to grab the item.