At various parts of The Brume Tower.
Nadalia, like Elana and Nashandra, is a fragment of Manus after the latter was defeated by the Chosen Undead centuries beforehand. She came to the Brume Tower in search of the Old Iron King, either out of genuine affection or to simply force his kingdom to crumble. But when she arrived, she found that the king had been slain, and his kingdom left in shambles.
Falling into despair, Nadalia split her soul apart and stored those fragments within various ashen idols throughout the hollowed and empty Brume Tower. Upon doing this, she erected a black fog that consumed the tower and brought back the deceased soldiers and warriors, bewitching them into defending her. Many knights of outside lands came to the tower to steal the stores of iron within the Brume Tower, but Nadalia's idols both bewitched the ones who were willing to serve her, and destroyed the ones who would attempt to defy her.
Sometime after his defeat at the hands of Velstadt, Sir Raime of Drangleic came to the Brume tower in search of greater power, and fell in love with Nadalia upon seeing her. Rather than purge the black fog, Raime instead chose to live alongside the fog, and became Nadalia's loyal guardian.
Her original body can presumably be found at the bottom of the tower past Raime, clutching the Crown of the Old Iron King and sitting upon his throne.
Nadalia is not a boss as the previous two Queens were; rather, she is more akin to an obstacle that is found throughout the Crown of the Old Iron King. The only way to destroy these idols is to impale them with the Smelter Wedges that the player finds throughout the tower.
Soul of Nadalia, Bride of Ash