ΔE 25.6, loose: different enough that you should pick on purpose. Bright Slate Grey is noticeably lighter, much more muted than Stormfiend.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Bright Slate Grey is noticeably lighter, much more muted than Stormfiend.
The same difference applies the other way round: Stormfiend is noticeably darker, much more saturated.
ΔE is the colour difference between our two swatch estimates: under about 6 is close, over about 15 is loose. Real paints also differ in finish, coverage and how they thin, so test before you commit.
See Stormfiend on the colour wheel See Bright Slate Grey on the colour wheel
All matches: Stormfiend equivalents · Bright Slate Grey equivalents