It's deliberately very non-normal harmony- they're adding extra notes to make deliberate clashes to give a mysterious mood.
The chord progression is based around F#m- Fm, which is fairly normal.
The notes in the progression you mentioned basically turn it into F#m maj7 b5 - Fm augmented if you want the technicalities. The principle is you add in some clashy notes (F against F# and C# against C) to make it sound dark and eerie.
