Blighter

Prerequisite: The ability to cast at least one spell

When you cast a spell that deals damage, you may choose to have it deal necrotic or poison damage in place of its usual damage type. Once per turn, when an enemy is affected by one of your spells of first level or higher that deals necrotic or poison damage, their hit point maximum is reduced by half the damage dealt until they complete a short or long rest. This reduction takes place after they are damaged—for example a creature with 30 maximum hit points who takes 10 necrotic damage would be left 20 hit points out of a maximum of 25.