You target a creature's arm or leg, and the creature makes a CON save.  On a failure, the creature takes 8d8 + 20 necrotic damage.  On success, the creature takes half damage.  If the creature fails the save by 5 or more, the targeted limb instantly shrivels, lesions spewing green smoke as the skin is ripped open, leaving desiccated flesh and bone behind.  The DM determines what effect the loss of limb incurs on the affected creature.  For example:

Arm. The affected creature loses all use of the targeted arm.  Weapons or items held by this limb fall to the ground, and actions or items which require both hands fail.  Strength-based skill checks which require the use of hands, such as climbing, grappling, and shoving, are made with disadvantage.

Leg. A creature which uses four or more legs has their walking, climbing, and swim speed (if any) reduced by 10ft.  A creature with less than four legs instantly falls prone, and its walking, climbing, and swim speed (if any) is reduced by half.

Spells like regeneration do not allow the limb to grow back without first conducting a Medicine check equal to your spell save DC, or casting a spell like remove curse. A wish spell can replace the limb.  

Casting this spell requires the caster to not only wish harm upon the creature, but to relish in horror and despair as the caster watches a creature's body wither to bone.  Non-evil casters who attempt to cast the spell may have their alignment moved closer to Evil, or have to atone to their Deity for committing this atrocious act.