It's just a temporary inconvenience, except in a singular case.
Basically, all that a familiar's death means is that you'll have to recast Find Familiar at your earliest convenience (so it means the loss of a spell slot for a day if you have a time pressure and can't take the extra 10 minutes to cast it as a ritual). The spell/ritual does cost 10gp, but that's a pretty trivial amount for an adventurer.
It also has a casting time of 1 hr so your party probably needs a short rest while you do it rather than simply doing it between encounters.
The only case where having a familiar come to harm is a permanent issue is in the rare event that you decided to retrain the spell (or as a wizard, have lost the spell book that contains it). These are the only times when losing a familiar has long term ramifications.
As to whether or not the newfound familiar obtained when you recast the spell is the same being reincarnated/resurrected, or a different being entirely, this is up to you and the flavor you choose for your familiar.
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
Its body disintegrates and a lower-ranking modron evolves to take its place.
In D&D 5th edition, destroyed monodrones are simply replaced with newly-built ones, and higher-ranking modrons are replaced by promotion of lower-ranking ones (Monster Manual, p. 224):
It's not clear whether modrons have a soul. They are constructs, machines. They are built, not born. However, in an earlier edition of the game, they are defined as living constructs, a type of construct which is sentient, can have free will, and can be raised or resurrected, suggesting they may have a soul. (Return of the Modrons, Dragon Magazine #354, p.35-49.)
There is some lore which suggests that their life energy could be trapped in that plane. In Return of the Modrons, p. 41, it states:
Hence, a modron's life energy returns to the Energy Pool from which Primus creates all new modrons, but the exact mechanism by which the energy returns is canonically a mystery. It's therefore unclear whether a destroyed modron's energy can escape Barovia, but this does not seem to prevent the creation of the modron's replacement. Since Primus controls all modrons (with some exceptions), it is unlikely that he would intentionally send enough modrons into Barovia to drain the Energy Pool to the point where new modrons cannot be created.