In my campaign, I used the Mire as a breeding ground for undead that the lich to the north had sent his captains down to get recuits for his forces. They encountered another lich that had taken up residence there and left in a hurry without to much exploring. Correctly me if I am wrong but the Mire is from a very long time ago, something like the end of the second age or the beginning of the next, like 6,000 to 3,000 years old, think of the cities of Egypt or Greece and they have only been deserted for 2,000 years.