The spell would effectively drain water and vitality from a creature, which would suffer serious necrotic damage. Undead and constructs were immune as they either didn't have or didn't need moisture. Plants would just wither and die unless they were plant creatures or a magical plant, in which case they would just have a disadvantage to resist the spell, If they failed, they would suffer the full damage of the spell.
If used with more skill, the spell would cause even more necrotic damage.
The spell had verbal and somatic components.