The Immortal origin means that a creature will no die to old age, but they can still be killed by other methods. From the compendium:
Immortal creatures are native to the Astral Sea. Unless they are killed, they live forever.
If the character would die a second time they would be dead until a Raise Dead ritual was used or some other method was used to raise them. The ability only happens once per day, once you use it to not die once in a day you cannot use it again until the next day.
Note that while this ED does make you very angel like, it does not actually make you an angel.
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.
[W]hat does Magic Circle even do if you cast it on an area that already contains an excluded creature?
Traps the creature inside. The spell says:
When you cast this spell, you can elect to cause its magic to operate in the reverse direction, preventing a creature of the specified type from leaving the cylinder and protecting targets outside it.
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
A traveler on the Ethereal Plane is invisible and utterly silent to someone on the overlapped plane, and solid objects on the overlapped plane don't hamper the movement of a creature in the Border Ethereal The exceptions are certain magical effects (including anything made of magical force) and living beings.
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
No general rule causes force damage to pass from one plane of existence to another. #DnD
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:
Simple answer are typically missed.
Always keep one party member on watch. If someone always keeps watch
(changing shifts so everyone gets full rest) they can look for party
members having nightmares. Having a nightmare of terrifying images
isn't a quiet thing -- ask any parent. Try putting the people with
highest perception on watch at the times you're most vulnerable, like
the middle of the night.
Once awake, the nightmare ends. If caught before an hour, the negative
effects don't take hold. The ability only works on sleeping people on
the material plane. The hags "Nightmare Haunting" is used up for for
that 24 hours.
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?)
Best Answer
Kill the hag, or obtain the bag some other way.
Unless Groody had a really poor relationship with the rest of the party, this is not how it's going to go down. We want vengeance, and we want to free our friend's soul.
Not even true resurrection can bring Groody's soul back from the hag bag, as even the most powerful resurrection spell requires the soul be both free, and willing. So the only solution is to free Groody from the soul bag the old fashioned way - kill the hag and destroy the bag. To be fair, the Night Hag's description doesn't tell us what happens in this scenario, but to me, it seems quite natural that destroying the soul bag would release the soul inside.
And then return to the priest with another fat sack of gold.
That said, any number of other methods may work here, as long as they involve the soul being released from the soul bag, be it through a trade deal with the hag, deceiving the hag, stealing the bag, etc.