Dragonspear Castle
Though the structure is crumbled and perpetually shrouded in mist, more than one of the caravans I've guided through these lands have seen Dragonspear Castle from afar and expressed a desire to seek shelter there. As I tell them at such times, it is better to seek shelter inside an opened tomb in these lands, and crawl in to huddle among the warrior dead within, than to seek anything like sanctuary from Dragonspear.
Built by an adventurer named Daeros after he found a wealth of gems in a sunken dwarven settlement, Dragonspear Castle was erected above the very caverns where that settlement—fallen Kanaglym—was interred. Two hundred years ago, sorcerous machinations brought about the fall of Daeros and the opening of an infernal portal in the depths of the castle.
After that event, Dragonspear's ruins were occupied by hobgoblins and myriad bands of bandits, until Waterdeep and Baldur's Gate sent troops to root them out. Discovering that the portal yet existed, but unable to destroy it, they established the Hold of Battle Lions, a fortified temple of Tempus, in an attempt to prevent anything from coming through. In time, though, devils broke through new portals inside the castle's walls and overran the defenders.
Then came the Second Dragonspear War, more than a century ago, during which a strange cloak of mist settled over the castle, and the forces of Waterdeep and Baldur's Gate once more attacked. This time, they defeated the devils, leaving the castle ruined and still cloaked in mists. At least one other time since then, the devils have punched back through, amassing other fell creatures to attack the nearby settlements (notably Daggerford), but they have been fought off by adventurers each time. The most recent stories tell of heroes confronting Red Wizards of Thay and other devilry. I pray that this will be the last time such efforts are necessary, but somehow, I think not.
Today, Dragonspear remains crumbled and mistshrouded. Rumors say that the castle—seemingly quiescent—has become home to undead horrors of some sort, but no one seems terribly inclined to investigate such claims, so long as they don't threaten the folk who live nearby. Some interested parties out of Baldur's Gate offered me more than a fair amount of coin to investigate the truth of these rumors, though I demurred. I don't fancy myself an investigator or a spy, and I know better than to seek out whatever foulness might have taken hold in this place.