The Shadow Marches (2004)

From the introduction:

Deep within the fetid lands of the Shadow Marches, an ancient shrine of the Gatekeepers rots in eternal ruin. Built atop a sealed portal to the plane of Xoriat, from which in distant memory horrid aberrations once emerged to wage brutal war upon the young nations of Khorvaire, the shrine is to this day a sinkhole of evil in the midst of the swamp, mutating nature itself to evil.
...
"The moist air of the fetid swamp seems to close in around you with the gathering gloom of evening, threatening to choke you with the thick odors of rot and bloom. For days you have crossed the trackless moors and uncharted bogs of the Shadow Marches, and now at last your destination is close at hand. The ancient shrine you seek hulks on the horizon, a ring of stones surrounded by a bog of foul water. The air hangs heavy with a brooding presence, as though the shrine—or perhaps the dragonshard you seek within it—were expecting you.

"You are to be well paid for this expedition; you have had that at least to comfort you through these endless days of slogging through mud and rain. Your employer, Tophran Demilek, is one of the wealthiest people in the great city of Sharn, and he has promised you a worthy bounty if you bring him the dragonshard he seeks.

"Now the air has grown still, seeming to mute even the noise you make as you slosh through shallow pools and tall rushes. You have arrived: The dragonshard must lie at the heart of that ancient circle of standing stones, just across that bog."