C-sharp major
The "Elektra" chord is a "complexly dissonant signature-chord" and motivic elaboration used by composer Richard Strauss to represent the title character of his opera "Elektra" that is a "bitonal synthesis of E major and C-sharp major" and may be regarded as a polychord related to conventional chords with added thirds, in this case an eleventh chord. It is enharmonically equivalent to a 7#9 chord : D-F-A-C-E and a 6b9 chord : E-G#-B-C#-F.