You fill a willing target with necromantic energy.   They gain the following benefits:  

• Immune to poison damage and the poisoned condition
• Resistance to necrotic damage and non-magical, non-silvered piercing damage
• Detect as undead and are affected by spells such as Protection From Good as if they are Undead. 
• Mindless undead ignore them unless they attack them. 
• Advantage on any persuasion or deception skill used against undead.
• Darkvision 60 ft
• Any Exhaustion effect does not penalize them for the duration of the spell.
• They do not need to breathe, sleep, eat, or drink while the spell is in effect.
• As a Bonus Action they can fire a Grave Bolt upto 60 ft.  It is a ranged spell attack that does 2d8+ Charisma Necrotic damage.
• Sunlight sensitivity - they have disadvantage on attack rolls and Wisdom (Perception) checks made in sunlight.
• If they die while the spell is in effect, or as a result of Exhaustion after the spell ends, they rise as a Deathlock Wight.

If cast at higher levels:  If cast at 4th level it lasts 1 hour.  If cast at 5th level it no longer requires concentration and lasts 1 day.