piano scale
C♯ Phrygian
C♯1D♭2E♭3F♯4G♯5A♭6B♭7
Minor with a flattened second — the half step right above the tonic is the whole character.
The same sounds spelled from the other side are D♭ Phrygian.
Written out
Every accidental is written in, rather than left to a key signature, so the letters can be read one at a time.
The step pattern
C♯HDWEWF♯WG♯HAWBWC♯
What is true of it
C♯ Phrygian is C♯, D, E, F♯, G♯, A and B, and back to C♯.
Its steps are H – W – W – W – H – W – W, where W is a whole step and H a half.
On the same tonic, C♯ natural minor differs from it by exactly one note.
Chords made only of these notes
Found by testing every chord in the catalogue against these 7 sounds, not chosen.