The Locrian mode is one of the most sonically challenging sounds in music. And, though built on the major scale, this mode sounds totally removed from its parent scale's bright, happy vibes.
Most often, the mode is used in heavy metal (Metallica's Sad But True being a good example) or as a way to improvise over the m7b5 chord (the ii chord) in a minor ii-V-i progression in jazz – so there's plenty to be gained from adding the mode's signature tense sound into your own playing.
