Flash of the Blade is a heroic tier feat.
Prerequisite: Rogue, proficiency with rapiers
Benefit: While wielding a rapier, if you are the only creature adjacent to the target you may use your sneak attack damage even if you do not have combat advantage against the target with this attack. [Dr381:70][U :3/2009]
Errata[]
This feat was revised in the 30th of March 2009 update. The original text was as follows:
Prerequisite: Rogue, proficiency with rapiers
Benefit: If you hit a target with a weapon attack while you are using a rapier and you are the only creature adjacent to that target, you can deal Sneak Attack damage to the target even if you do not have combat advantage against that target.