Thuria is a ruined city in the small province of Darandis.
It is said that evil forces have claimed the city.
As Heriad (the inn-keep at The Lucky Fox Tavern) tells it:
Thuria used to be the main city in Darandis. It was bigger than Erech. A couple hundred years ago wizards in the city were experimenting with magics, and accidentally released dark forces unto the city through some sort of portal or fissure. Demons began destroying the city. A group of good wizards created a protective bubble around the city that held the evil captive within the city limits. Since these events, no one entering the city has ever been seen again, and strange happenings have been reported in the vicinity of Thuria.
This is also the main reason why people in Darandis are not particularly taken with magic-users.