Blood Boil

The blood, ichor, or other life-sustaining fluid inside a creature heats to intensely painful levels. The target must make a Constitution saving throw. It takes 3d6 fire and 3d6 necrotic damage on a failed save, or half as much damage on a successful one. On a failed save, the target has disadvantage on attacks until the beginning of your next turn, and on subsequent turns, you may use your action to repeat the effects of the spell. If you use your action to do anything else, the spell ends. The creature repeats its saving throw as normal, ending the spell of a successful save.

If you have a sample of the target's blood, they have disadvantage on their saving throws.