Sounds like your DM is confused, or has given you a power that is normally only found on potent monsters.Hmmm, my DM told me the other day that my necrotic damage I was doing not only did damage but also lowered the targets max hps that can't be healed until they took a long rest. Basically even if they could heal themselves they could not heal the necrotic dmg I did until the next day. I can't find a source other than him for this rule.
This. There are a bunch of undead creatures in the MM that can reduce a target's hp max with their necrotic damage-dealing attacks, but that doesn't mean that necrotic damage does that inherently.It isn't an inherent trait of necrotic damage to reduce maximum hit points, it's the other way around - attacks that reduce maximum hit points almost always happen to do necrotic damage.