You’re talking about a certain vibe and a lot of that creepy and sinister vibe is going to come down to production and recording techniques. Like think about the beginning of shepherd of fire when you hear the crackling of the fire, the wind blowing, that super low tuba note, the bell tower. Its all reminiscent of a dark, hellish theme. Think about sounds in an abstract way and then figure out what it takes to make it happen, but ALWAYS imagine a sound in your head first.
The in terms of musically what you can do on your instrument is give your self a slow to moderate tempo. Like the opposite of “Trashed and scattered”. As much as I love that song, it’s not exactly what I would consider “sinister”. Give your riffs space and find a solid groove or 2 to build off of. Make use of lower notes and tones, again, like that tuba in shepherd of fire.
Spending a little time on the b5 rather than using it as a passing tone can help. Nightmare does this in that intro melody. The b2 can be used, although it may be hard if your ear isnt used to it. Like Jak said, chromaticism can be you friend. Find your main tonality and like B minor, and then start using all of those notes outside of the scale and resting on them a little bit and deciding what feeling they give you against a chord progression in B minor(or whatever key youre in) or just a B minor vamp recording.
And finally more or less disregard that last paragraph and just let your mind and ear figure it out! Think of creepy sounds and melodies and THEN use your instrument to bring them to life. Imagine, hum, sing, whatever you gotta do- just make sure the sounds are coming from within!