Queen Filfaeril's Blades was a secret society of spies and assassins working to maintain order within the kingdom of Cormyr.
Under Toraunt Hawkgar, the Blades did their best to keep a low profile, as rumours of their activities made it more likely that their identities would be revealed by the War Wizards, whose attentions they avoided to the utmost.
The Blades killed or otherwise disrupted the plans of seditious citizens and nobles who worked against the Crown. This included smugglers and tax evaders as well as those guilty of high treason. They also surreptitiously fostered trade alliances between other nobles, endeavouring to get their targets to make beneficial deals publically rather than in secret.
The Blades worked towards Queen Filfaeril's vision of Cormyr, a vision they dubbed "Fee's Fire". In it, a kingdom is believed to be most secure when it's people are busy and prosperous and that those who know they will be rewarded for doing so, are more likely to be law-abiding.
In other words: Commoners must be made to see that committing crimes or being elevated to the nobility are not the only methods of personal gain. Therefore, nobles cannot be allowed to exert their authority to the detriment of the common folk under their purview and the coin earned by the realm should be distributed as a fair wage to those who've worked to make the realm prosperous.
Some time after Filfaeril Selazair became Filfaeril Obarskyr, the queen of Cormyr, she realized that she had a political opponent in the form of Vangerdahast, whom, it quickly became clear, thought himself the most capable individual to effectively rule the kingdom. To ensure that her husband Azoun maintained the power in his own realm, she organized a group of agents who would operate in complete secrecy. They would do with their actions what Filfaeril could not with her voice (and the manipulative woman could do a lot with her voice).
With the help of Storm Silverhand and Dove Falconhand, Filfaeril weaved a web of misdirection to convince the more wily courtiers of the realm that her agents were all Harpers and Cormyrean Highknights loyal to her instead of the king. In reality, her Blades were her arm against internal threats to the Crown and she managed to keep their identities and doings secret from her husband as well as Vangey, her sister Laspeera and the other War Wizards.
During her life, the Blades managed to gain knowledge of several potentially devastating noble rebellions and stop them by performing strategic assassinations against the rebel leaders. When the rebels saw the instigators of their seditious movements slain in mysterious circumstances, their self-interest tended to take precedent over their desire for regime change.
After Queen Filfaeril's death in 1390 DR, the leader of the Blades at the time, Anathur Hawkhorn, kept the group going by reporting the results of their missions to Filfaeril's personal crown (the tiara she wore day-to-day, not the state crown). The other Blades began to believe that their sponsor's spirit could observe them through the crown and Hawkhorn's successor continued the practice.