by Of Monsters and Men
is most likely in the Key of C#/Db Major
Notes in C#/Db Major:
A#/Bb, C, C#/Db, D#/Eb, F, F#/Gb, G#/Ab
Chords in C#/Db Major:
Db, Ebm, Fm, Gb, Ab, Bbm, Cdim
This key can also be known as A#/Bb minor.
When it's used as a minor key, the minor key note is stressed more than the major key note. The same notes and same chords still apply.