Let's Summon The Devil is written in the key of C Locrian.
According to the Theorytab database, it is the 3rd most popular key among Locrian keys and the 75th most popular among all keys. The C Locrian scale is similar to the C Minor scale except that both the 2nd and the 5th notes (D♭ and G♭) are a half step lower. This means that the characteristic "home base" chord of C Locrian has a dissonant diminished quality (c˚), which makes it a very uncommon choice for songs.
See the C Locrian Cheat Sheet for popular chords, chord progressions, downloadable midi files and more!