Harm
You unleash virulent magic on a creature you can see within range. The target makes a Constitution saving throw. On a failed save, it takes 14d6 Necrotic damage, and its Hit Point maximum is reduced by an amount equal to the Necrotic damage it took. On a successful save, it takes half as much damage only. This spell can't reduce a target's Hit Point maximum below 1.
Harm 5e 2024 vs. 2014 edition differences
The 2024 version clarifies that damage reduces the hit point maximum by the same amount as damage taken, and explicitly states the maximum reduction can't reduce HP below 1. The 2014 version states the maximum HP reduction lasts 1 hour and can be reversed by effects removing a disease; the 2024 version omits this duration and disease removal detail.