The Violet Nebula is a nebula located on the antispinward side of the Kalandra sector. The nebula's composition blocks sensors and communications. It was mostly avoided due to the gravitational eddies within, which were a danger to starships.
During the Kalandra Campaign the Federation Alliance lured the remains of two Dominion strike fleets into an ambush at the nebula, destroying them. (The Dominion War Sourcebook: The Fires of Armageddon)