Question about which saving throw to use

[KittenEV]

Hi, I'm throwing a Night Hag at my level 3 players. I have 6 players in the party so it shouldn't be too difficult for them to take down. My question is for her Nightmare Haunting. It says that she just touches them while in the Ethereal Plane and their hit point maximum is reduced. Now, I've done this once to one of my players already, but no one in the campaign discussed it. If I keep doing it to that player however, (which is what the hag would do) that player might not be able to fight when the time comes since she'd have such low hit points.

Because of that, I was thinking about making my player roll a saving throw when it happens to make the Nightmare Haunting less op. But, which kind of saving throw, do you think would be apprioriate. It's a Nightmare so I thought Wisdom, to you know see through it, but maybe Constitution because its bringing her max hit points permanently down?

What do you guys think?

[Vaz]

Reducing max hp is an awful mechanic. Don't do it, is my first bit of advice. My second and third also.

Especially at this low level. Best thing to do is instead do damage and make their next long rest have no effect, giving them a point of Exhaustion.

That said Wisdom has traditionally dealt with Sanity and Nightmares.

[Cespenar]

Make the reduction curable by Lesser or Greater Restoration.

The save could be Wisdom or Charisma, representing the spirit.

[Theodoxus]

Another option, to reduce the deadliness, is to give it two saves. Wis to notice the nightmare, negating the effect, and if that fails, a Con to save against the drain, granting resistance (1/2 damage) to the effect.

Might boost one or the other save DCs if you want it to still be relatively harmful - determine where on the sliding scale you want the effect to be, and then stick it there. (I wouldn't tailor the save to that specific character though. If she has proficiency in Wis, but not Con, I wouldn't necessarily make the Wis harder because she's more likely to make it...)

[Contrast]

Couple of options come to mind.

Treat it as a condition - once applied you cannot be the target again until the condition is removed.

Another option is to reduce the ethereality - make the hag ghostly but visible. So it'll work if someone sleeps on their own or if everyone is asleep but if someone stays awake on watch they can ward against it.

Have them find a holy/magical site with a permanent magic circle effect. Its inconvenient to go there to sleep but it protects them.

You say no-one made much of a fuss with the previous. Are they aware in character that these abilities are a thing? Maybe drop that knowledge on them in character somehow.

[Plaguescarred]

I'd go with a Constitution saving throw since it measures health, stamina, and vital force, which is what it drains.