You touch a creature with necromantic energy.  Make a melee spell attack.  On a hit, the creature must make a Constitution saving throw.  The target takes 9d12 necrotic damage on a failed save and half as much on a successful one.  Additionally, on a failed save the creature is infected with a disease that slowly drains them of their life over the course of twenty four hours.  With every six hours that pass, they will gain one of the following conditions: 

6 Hours- The target begins to feel weak in their limbs as their muscles begin to deteriorate.  They now have disadvantage on all Strength checks and saving throws.

12 Hours- The target begins to loose focus as their cognitive functions begin to slow.  They now suffer a -3 to all mental ability scores  (Intelligence, Wisdom, Charisma).

18 Hours- The target begins to resemble a walking corpse.  They now suffer a -9 to all physical ability scores (Strength, Dexterity, Constitution).  Additionally, the target now has half of their original health as their new maximum.

24 Hours- The target has succumb to the spells full effect and has died.