ΔE 20.7, loose: different enough that you should pick on purpose. Warlord Purple is noticeably lighter, much more saturated, cooler than Hastur Purple.
Not as a straight swap; see the closer alternatives below. Loose match (ΔE 21). Warlord Purple is noticeably lighter, much more saturated, cooler than Hastur Purple.
The same difference applies the other way round: Hastur Purple is noticeably 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 Hastur Purple on the colour wheel See Warlord Purple on the colour wheel
All matches: Hastur Purple equivalents · Warlord Purple equivalents