ΔE 25.9, loose: different enough that you should pick on purpose. Neptune's Fury is a little lighter, much more saturated, cooler than Cultist Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 26). Neptune's Fury is a little lighter, much more saturated, cooler than Cultist Purple.
The same difference applies the other way round: Cultist Purple is a little darker, much more muted, warmer.
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 Cultist Purple on the colour wheel See Neptune's Fury on the colour wheel
All matches: Cultist Purple equivalents · Neptune's Fury equivalents