Redwhait
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Because immunities related to the "Immunity to Mind Magic" ability (which undead creatures have) are removed by the spell, but those related to the "Undead" ability itself are not (for example, immunity to curses is not removed). At the same time, immunity to blindness is granted by both abilities – and therefore, immunity to it is preserved when using the spell due to the second ability. Troglodytes also retain their immunity when using the spell, by the way. However, for titans and elementals, whose immunity to blindness is granted only by the "Immunity to Mind Magic" ability, the spell will remove the immunity to blindness.