This question was asked previously with respect to D&D 3.5:
What are the ways that a CR-appropriate party can stop a Night Hag's dream haunting?
There are some interesting answers there, but none of them were selected as the answer, and the monster has changed between editions. In particular, that question notes that Night Hag's CR was 9 in the previous edition, whereas currently they are CR 5. Presumably, a full-strength 5th level party of four adventurers should be able to defeat a Night Hag.
The Night Hag has the following action:
Nightmare Haunting (1/Day): While on the Ethereal Plane, the hag magically touches a sleeping humanoid on the Material Plane. A Protection from Evil and Good spell cast on the target prevents this contact, as does a Magic Circle. As long as the contact persists, the target has dreadful visions. If these visions last for at least 1 hour, the target gains no benefit from its rest, and its hit point maximum is reduced by 5 (1d10). If this effect reduces the target’s hit point maximum to 0, the target dies, and if the target was evil, its soul is trapped in the hag’s soul bag. The reduction to the target’s hit point maximum lasts until removed by the Greater Restoration spell or similar magic.
Let's assume that a 5th level party has already had a member suffer one night of Nightmare Haunting, and through player knowledge, role-played interactions, or Arcana checks, they've figured out they're facing a Night Hag and are expecting night two. Let's also assume they have all the knowledge of the Night Hag MM entry, knowledge of all the spells in the PH (and a mix of caster's who can prepare any spell up to 3rd level), and no magic items.
Contact can be "prevented" by:
- Protection from Evil and Good, which is 1st level, but only lasts 10 minutes.
- Magic Circle, which is 3rd level, and lasts an hour.
But since the Night Hag can come any time during the night, it's not clear that these spells are helpful. Magic Circle lasts longer if cast at higher level, but it doesn't help our 5th level party, and depending on your DM's sleep rules (not well-covered in RAW), may not be helpful until you get to an 8th or 9th level slot.
So in order to "prevent" contact, you wait for the Night Hag to arrive, then you cast the spell. But how does the party even know the Night Hag is there? See Invisibility can be used to see ethereal creatures, but it only lasts an hour, and the Night Hag can come any time during the night.
Then, if the party realizes the Night Hag is there but the Nightmare Haunting hasn't started, a spellcaster casts Protection from Evil and Good or Magic Circle. So the Night Hag goes away, and comes back after the spell expires. Repeat until no more spell slots, or at least deprive most of the party of their long rest.
So the party realizes it's fruitless to drive the Night Hag off before the Nightmare Haunting has begun. The spellcaster waits for the Night Hag to begin Nightmare Haunting a sleeping party member, then casts Protection from Evil and Good or Magic Circle. The Night Hag's 1/day power is spent. But do these spells even work? It's not clear to me that "prevented" means "stops" in this context. (And what does Magic Circle even do if you cast it on an area that already contains an excluded creature?)
In fact, the Night Hag still seems pretty challenging for higher level parties when used this way. Greater Restoration at least removes reduction of HP maximum, but killing the long rest can allow the Night Hag to systematically weaken a party. Etherealness lasts 8 hours and will eventually allow the party to post a sentinel in the Ethereal Plane (and hope the Night Hag doesn't kill the sentinel).
Please help before I TPK my players.
Best Answer
Magic Circle and Protection from Good and Evil
If the Hag always finds the party about the same time of night, they could cast either in preparation for the hag to keep the Hag out. Protection would be better, as the Hag might forget to detect magic and waste the casting on an attempt to touch someone who is already protected.
Protection from Evil and Good is only an action to cast, so it can be cast easily even when the attack has started.
Traps the creature inside. The spell says:
So could trap the Hag -- but the problem here is that the casting Magic Circle takes 1 minute. Unless the DM allows you to hold completion of a spell similar to a ready action -- But that isn't RAW.
Oil of Etherealness
Applying the oil takes 10 minutes, and it lasts 1 hour. It will allow you to be on the ethereal plane. If you fight and break the touch the Hag, the Haunt attack is used for the day. If you secure the Heartstone, she can't get back to the ethereal plane, and if you kill the Hag, she'll be banished back to Hell.
Turn the Hag
Paladins of the Oath of Devotion's Turn the Unholy works when the Hags enters the material plane to attack.
Paladins of the Oath of Ancient's Turn the Faithless doesn't require Sight, if the Fey or Fiend hears the chanting they have to flee.
Paladins for the Oath of Vengeance can frighten a fey or fiend they see using Abjure Enemy.
Force Damage Won't Work, but Things of Force Do
However closer reading is that things made out of magical force hamper movement -- force damage doesn't pass through realms. This is made clear by a tweet from Jeremy Crawford:
https://twitter.com/JeremyECrawford/status/745062527118123009
So while Magic Missle or Eldritch Blast won't help, the following will: - Leomond's Tiny Hut - Wall of Force - Force Cage
Wake the Sleeper Will Not Work
Originally I thought that waking the sleeper would work. I said:
However, RAI based on this tweet from Mike Mearls, that isn't the way it works. The victim only has to be asleep to start the nightmare haunt, but waking doesn't end it.
Alarm Won't Tell You She's coming
I thought Alarm might give you some warning, but its effect appears to be limited to the material plane. (Unless you cast it while in the Ethereal?)