Basillus was a cleric of Cyric who went mad after his family died.
Originally from Zhentil Keep, Basillus left the Moonsea after the tragedy that claimed the lives of his family members and wound up living in the Sword Coast. In the Year of the Banner, 1368 DR, Basillus murdered a significant number of innocent people. He raised them as undead zombies and skeletons, believing them to be the family which he had lost before.
That same year, Governor Kelddath Ormlyr of Beregost placed a 5,000 gp bounty on Basillus' head. He tasked those who sought to stop the murderous cleric with the return of his holy symbol.
While it was never explicitly stated, it was quite possible that Basillus' family perished in the Siege of Zhentil Keep of 1368 DR.