The Harper schism is an event that occurred in 1370 DR in which the Harpers uncovered apparent dealings between the Zhentarim and one of their highest members, Khelben "Blackstaff" Arunsun. This accusation caused a political division to form within the Harpers and heightened paranoia and mistrust, forcing many of its members to choose their loyalties.
In 1370 DR, the Twilight Trio called a meeting of the Harper Council. The trio consisted of Bran Skorlsun, Obslin Minstrelwish and Belhuar Thantarth. Cylyria Dragonbreast, the leader of the Harpers of Twilight Hall, was absent due to illness. They called Khelben "Blackstaff" Arunsun forward and accused him of treachery by having dealings with the Zhentarim. Khelben admitted his guilt and announced he was quitting the Harpers, along with his wife, Laeral Silverhand. His admission led to speculation that he was guilty of other betrayals, and the Twilight Trio spread word amongst the Harpers that he was a traitor.
The exit of Khelben from the Harpers and the subsequent investigations by the Twilight Trio caused a lot of resentment amongst its members, many of whom were strongly opposed to the trio prying into their affairs. Cylyria Dragonbreast was unaware of the trio's investigations until they had already begun, but set about trying to make amends as soon as she made the discovery, although her continuing illness prevented her from gaining much success.
By 1371 DR, three distinct partisan groups had formed within the Harpers. Some believed that the entire incident was not a worthy cause for the events that followed, and wished to play down the entire affair. Others had cause to trust Khelben's motives, believing that he would reveal himself when the time was right. Still others believed Khelben to be a traitor and that his admission of guilt was an indication of a more deep-rooted problem.