FamousHalloween1978 · John Carpenter
Main title, the 5/4 piano ostinato over held tonic notes in low brass and electronic strings
This is not a strummed chord but the three notes of Carpenter's piano riff: F♯ as the floor, C♯ a fifth above it, and D a minor sixth above, tapping out in 5/4 over a held low F♯. With no third it sounds bare and cold, and the D leaning a half step above the C♯ gives it the sting. Later the whole pattern slips down a semitone to F, C and D♭.
Also called Halloween theme ostinato, Michael Myers theme
Sources: Michele Darling, ''Halloween' Theme: What Makes the Music so Scary?', Berklee Online Take Note (30 Oct 2021) · Lisa Witt, 'How to Play the Theme From Halloween', Pianote (updated 12 Jan 2023) · Hooktheory TheoryTab: Halloween Theme · Hypebot reprint of the Berklee Online article