ΔE 20.4, loose: different enough that you should pick on purpose. Medium Violet is a little lighter, much more saturated, warmer than Naggaroth Night.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 20). Medium Violet is a little lighter, much more saturated, warmer than Naggaroth Night.
The same difference applies the other way round: Naggaroth Night is a little darker, 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 Naggaroth Night on the colour wheel See Medium Violet on the colour wheel
All matches: Naggaroth Night equivalents · Medium Violet equivalents