ΔE 22.1, loose: different enough that you should pick on purpose. Xereus Purple is a little darker, much more saturated, warmer than Necromancer Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 22). Xereus Purple is a little darker, much more saturated, warmer than Necromancer Purple.
The same difference applies the other way round: Necromancer Purple is a little lighter, much more muted, cooler.
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 Necromancer Purple on the colour wheel See Xereus Purple on the colour wheel
All matches: Necromancer Purple equivalents · Xereus Purple equivalents