Sharing some info from the D&D sourcebook Descent Into Avernus (page 162):
Spoiler warning! This section reveals details about the story of Baldur's Gate 3.
Once a vigorous and formidable politician, Duke Belynne Stelmane recently suffered a seizure that left her with a partially paralyzed face and slowed speech. In truth, a mind flayer provoked the duke's "seizure" when it took mental possession of her. Now Stelmane wages a silent war against the mind flayer's influence, biding her time until she can find a way to signal for aid or regain her will. Not even Stelmane's aides are aware of her secret struggle, though they cover for her as best they can. Given her current situation, Duke Stelmane is in no position to oppose attempts by her fellow dukes to seize the reins of power in Baldur's Gate.