Curse Blood

You magically cut yourself on the wrist, taking one point of piercing damage, and extract a bit of your own blood from your body. You cast this blood at any creature you can see, thereby cursing their blood. This creature must succeed on a Constitution saving throw or take 4d12 necrotic damage on a fail and half as much on a save. On a success, the creature only takes half damage and is not cursed. On a fail, the creature takes full damage and is cursed for one minute. On the start of each of their turns for one minute, they must succeed on the saving throw or take an additional 4d12 necrotic damage. While a creature is cursed, they have disadvantage on attack rolls, and all attacks against the creature have advantage. Additionally, when a creature scores a critical hit against this creature, instead of dealing double the damage die, the creature takes 3 times the damage die.