ΔE 24.2, loose: different enough that you should pick on purpose. Graphite Grey is noticeably darker, much more muted than Stormfiend.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 24). Graphite Grey is noticeably darker, much more muted than Stormfiend.
The same difference applies the other way round: Stormfiend is noticeably lighter, 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 Graphite Grey on the colour wheel
All matches: Stormfiend equivalents · Graphite Grey equivalents