If you want spooky riffs and melodies, this will depend on the chord progressions you use. As for the melodies, make sure to use descending chromatic halfsteps (I'm not sure how much knowledge you have in theory) at the most focal points of the melody to really give off that sound.
- Good chord progressions in minor for spookiness:
3-2-4-1 (in a minor key), 2-1-3-2, 1-5-6-2, etc etc
The idea is to jot down a spooky series of root notes and then work out the chords that fall under them given the key, the inversions that fit them, etc, although, you'd probably want to use barre chords, especially in drop d (where three notes on the same fret play the first and fifth degree)
- Use dyad chords with ghost notes (x) that skip a string for added spookiness. This is what makes 'God Hates Us' and 'Natural Born Killer' sound so spooky.
- Chromaticism (playing notes outside of the seven allowed by the key) in melodies creates huge dissonance.
For example, the intro of the Wicked End is full of chromaticism, as numerous notes are not in the key of D Minor (such as the G#/Ab).
The ideal prospect is to blend in the sad and emotional with the spooky as Syn does. Also, switching between relative major and relative minor smoothly is a really valuable feature of composition.
*Chromaticism isn't required per se for spookiness, but certainly helpful