You touch a creature, forcing it to make a Wisdom saving throw. On a failed save, roll a d6 and consult one of the tables below based on the current situation (combat or noncombat). If the creature fails the saving throw by 5 or more, you choose the curse.

Combat (d6):

1. The creature’s sex changes for the duration, and is Incapacitated until the end of its next turn. If the creature has the Immutable trait or lacks a biological sex, your curse has no effect.
2. The creature becomes cowardly, having Disadvantage on saving throws against the Frightened condition and Morale checks (optional DMG rule).
3. The creature avoids lethal force, attempting only nonlethal attacks with its melee attacks.
4. The creature is Blinded or Deafened (your choice). 
5. The creature’s nonmagical armor deteriorates, reducing its AC by 2 (or by 1 if its AC is 11 or lower).
6. The creature polymorphs into a random Beast (CR 1/8) for 1 minute, as per the Polymorph spell. There’s a 40% chance the transformation occurs whenever the creature rolls Initiative.

Noncombat (d6):

1. The creature’s appearance and odor become foul, giving Disadvantage on Charisma checks.
2. The creature loses the ability to heal naturally and cannot spend Hit Dice.
3. The creature loses any Darkvision and becomes nearsighted, gaining Disadvantage on Wisdom (Perception) checks.
4. The target finds it difficult to lie, as if affected by a permanent Zone of Truth.
5. Beasts avoid the target and refuse to obey commands. It gains Disadvantage on all Wisdom (Animal Handling) checks. 
6. The creature gains 1 Exhaustion level until it consumes 1 pint of blood. If the spell is cast using a level 6+ spell slot, the creature becomes dependent on blood, gaining 1 Exhaustion level after each Long Rest until it drinks a pint of blood to remove it. Regular food and drink no longer satisfy the creature. If the target dies while cursed this way, there is a 25% chance it raises from the dead 24 hours later as a Vampire Spawn.

In combat, a creature can repeat the saving throw at the end of each of its turns. On a success, the curse ends. If the target fails three of these repeated saving throws, the curse lasts for the duration. The curse can otherwise only be removed by Remove Curse or Greater Restoration if cast with a spell slot equal to or higher than the one used for this spell.

Using a Higher-Level Spell Slot. The duration increases: 8 hours (4th), 24 hours (5th), 7 days (6th), 30 days (7th), or Permanent until dispelled (8th+).

Adapted from some Dragon magazine articles