Locrian guitar scale
See the Locrian scale on a guitar fretboard, learn its notes and degrees, and practice it in any key.
What it is
The darkest and most unstable mode: a flat 2nd and a flat 5th, so even its home chord is diminished. Rarely used as a home key.
How to use it
Play it over m7♭5 (half diminished) chords, for example the ii chord of a minor ii to V to i. Treat it as a colour for one chord, not as a key to stay in.