Water Siphon

Melee Spell Attack: +7 to hit, reach 5 ft., one creature. Hit: 21 (6d6) necrotic damage. The target must succeed on a DC 14 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken and it is stunned for 1 minute and gains one level of exhaustion. A stunned creature repeats the saving throw at the end of each of its turns, ending the stun on itself on a success. The hit point reduction lasts until the creature finishes a long rest and drinks abundant water or until it is affected by greater restoration or comparable magic. The target dies if this effect reduces its hit point maximum to 0.