ΔE 17.2, loose: different enough that you should pick on purpose. Nightmare Purple is a little darker, much more muted than Xereus Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 17). Nightmare Purple is a little darker, much more muted than Xereus Purple.
The same difference applies the other way round: Xereus Purple is a little 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 Xereus Purple on the colour wheel See Nightmare Purple on the colour wheel
All matches: Xereus Purple equivalents · Nightmare Purple equivalents