piano scale
D♯ major
The seven-note scale everything else is measured against: two whole steps, a half, three whole steps, a half.
The same sounds spelled from the other side are E♭ major.
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
What is true of it
D♯ major is D♯, E♯, F𝄪, G♯, A♯, B♯ and C𝄪, and back to D♯.
Its steps are W – W – H – W – W – W – H, where W is a whole step and H a half.
Written correctly it needs 9 alterations, F𝄪 and C𝄪 among them. That is what the letters force, and it is also why nobody writes music in this key: the same notes spelled from the other side are easier to read.
On the same tonic, D♯ melodic minor differs from it by exactly one note.
It cannot be written as a key signature at all, because a signature has no room for a double accidental.
Chords made only of these notes
Found by testing every chord in the catalogue against these 7 sounds, not chosen.