A ruin of some sort out in the marshes on the southern edge of the Brtov Sea. You will need to take small boats of some kind to explore it. Briste was an old anchor between the Land of the Wild, the World of Secrets, and other planes, and might be able to be revived with the right ritual.
You trudge up the stone steps for quite a ways, as you eventually emerge above the sloped stone wall on flat ground. It was imposing when viewed through the marsh trees, but now that you're standing atop it, it feels sprawling.
In the middle of this platform, a rectangular stone building with a sloped roof towers above you, intricate decorative patterns covering the bricks. Two stone doors lie directly before you, reaching almost to its roof.
Inside:
You push the massive stone doors open--some sort of mechanism in them makes this lighter and gentler than you expect. The weathered stone doesn't line up as well as it once did, making a noticeable scraping sound.
The room inside is larger than you expect, mainly because it reaches down into the depths of the pyramid-like structure. A sort of balcony lies just inside the door, with a set of stairs leading down the width of the room. You can't see much in the depths below you, aside from flickering torchlight illuminating something below, and the sounds of movement echoing from below.
At the bottom of the stairs, there is a large room, lit with magical torches which bathe everything in an orangeish, flickering light. Eight magical circles are engraved on the floor, and pulse with faintly glowing energy. On the far end of the room, a wide set of stairs rises up to a large room inset in the wall. Deep within that room, there is a pair of concentric stone rings etched in runes, floating just a couple of feet off the ground and rotating as magic circles are activated.
Six of the circles form a semicircle arc around these stairs; the other two are inset at the corners of the room nearest the staircase you descended.