At the risk of thread-snatching I'm very curious to know how this went! Did you play it as-written? How did they even defeat that encounter? Did the baddies just slip into the ethereal at the end to escape? (If we don't want to put too many spoilers in this thread please feel free to PM me!)
SPOILERS
...
So, firstly, the party was 6x 3rd level PCs, and they had a good balance of classes. Barbarian, fighter, monk, druid, cleric, warlock. All experienced gamers with competent builds (not optimized, but still pretty good). No magic items at all. Not even a
potion of healing. I tend towards low magic in all my campaigns, and in Ravenloft especially. The barbarian had a silver short sword (picked up in Death House), but that's it. They only found some of the magic items in Death House (the scrolls in the hidden library room), and refused to take them because they considered the scrolls belonged to the kids (Rose and Thorn)!
So, under-equipped party... but over-sized. Six PCs instead of four; no magic weapons or potions. Fortunately, they knew what they were getting into. They had observed Morganthe in town, and knew she was snatching kids. So, there was no "knock on the door and have a chat with the nice old lady". They knew it would be a combat, so positioned themselves accordingly. They kicked in the door and started ransacking the ground floor. They kicked over the cauldron and, for effect, I had the dretch demon "spill" out of it. Of course, a single dretch is easy meat for a party this size and it died in the first round.
At this point, Morganthe appeared at the top of the stairs. I ruled that it took her a double-move (i.e. she "burned" her first round) simply getting into position; primarily because I knew it was still going to be a hellaciously difficult fight. This gave every PC a chance to act before she dropped the hammer. Now, most of the characters are melee types who were actually going to have difficulty ascending the stairs and attacking in the first round (stairs = difficult terrain, and they didn't have the speed to close with a single move). However, the druid had an answer for this:
thorn whip. He tagged Morganthe and pulled her off the stairs. That brought her rapidly down to ground level (with +1d6 falling damage, and prone as well). The melee types immediately piled in... finding to their dismay that she had resistance to all their non-magical attacks. Also, a really decent AC. The barbarian switched to his silver weapon and raged. They began to realize this would be a lethal fight.
Still going gentle on the group, I opened with a claw attack against the nearest combatant... and missed. Lucky for him. 3rd level monks don't have great HP. They hit her a few more times, but she had a ton of HP (and still taking 1/2 damage from most attacks). The cleric's
sacred flame cantrip wasn't working against her magic resistance either. But that's when the warlock shines. He dropped the inevitable
hex /
eldritch blast combo, with the invocation that allows him to add Cha bonus to damage. Suddenly, Morganthe is getting tagged every round (good attack rolls) for 1d10+1d6+3 force damage, with no resistance. Ouch.
And then things got
really bad for her. The druid dropped
moonbeam. 2d10 radiant damage per round, save for half. Magic resistance, right? But night hags have the shapechanger property, which means they roll saves against
moonbeam at disadvantage. The two cancelled out, and she was now taking (basically) an average of "ongoing 10 damage", plus the warlock's hexed blasts, plus a bunch of meatshields pounding on her.
The gloves now come off. As she's part of a coven, she has all the coven spells. I could've
counterspelled the
moonbeam, but I thought that'd be a cheap shot given how few slots a 3rd level druid has available, and how proud the player was that he remembered it was extra-good against shapechangers. However, it didn't stop me from hitting back with heavy stuff. The druid copped a
phantasmal killer - failed first save, but made the second. Thanks to WotC errata (subsequent saves are made at the
end of the PCs turn, after they take the damage) he took 4d10 psychic. That dropped him from max to 1 hp. It also broke his concentration on the
moonbeam. The following round, two of the fighter-types took a
lightning bolt. This was a rude surprise for the raging barbarian, who took full damage from the spell. The party fighter was dropped (healed the following round).
By this time, though, Morganthe was pretty hacked up. She got off one more attack, then succumbed to the damage. The party breathed a sigh of relief. The fighter received a
cure wounds to get him back on his feet, and they prepared to loot the room.
Then the other two hags appeared at the top of the stairs. No short rest (important - keep the pressure on!), no time to muck around. At this point, my players requested a quick break to change into their back-up brown trousers. They had just spent seven rounds beating down
one hag, and had burned more than half their spell slots (and a lot of hp). They were in real trouble now.
Again, I cut them a small break in that the hags didn't attack in the first round. Every PC had a chance to act before they started laying it down. With one hag dead, the others no longer had access to coven spells... but they still had their own innate night hag spells. They opened with
sleep (surprisingly useful against low level parties), and knocked out the injured fighter. Then it was a mix of
magic missiles and claw attacks. The
magic missile was the biggest problem for the PCs, because it's absolutely reliable. No mages in the party, so no
shield spells. Also, to really keep the pressure on, I was splitting missiles and focusing on injured PCs. Example: put one missile on any spellcaster using a Concentration spell, and the other two to wear down the hp of a meatshield. If someone gets dropped and then healed, put 1-2 missiles on the newly-healed PC (to knock them other again) and the other missiles on fresh targets.
This combination proved brutal. At any time the
magic missiles (or
sleeps) looked like they would cause a TPK, I'd switch to claws for a round or two, just to balance the danger level. The claws, at least, had a chance of missing (and frequently did). But it was still touch-and-go. By the end of the combat, the barbarian had used both rages, the monk had used all ki points, and every caster had used every spellslot. 2 characters ended the fight unconscious and bleeding out (and had to be manually stabilized). 5 spellslots had been burned to "kick up" fallen PCs, who usually ended up back down again quickly because of my
magic missile tactics.
At the end of the combat, two things had been achieved: the players were sweating profusely at the implied lethality of the encounter, and they were celebrating like maniacs for having beaten it without PC deaths. We run a high casualty count in our campaigns. We play frequently (1.5 x 5 hour sessions per week), and the group has lost over 80 characters (permanent deaths) since 5e came out. They knew this one could have easily ended in a TPK.
...
So why didn't it? A couple of reasons, from the DM's perspective.
1) They had numbers (six PCs) and that's an enormous help. It means an extra few spellslots, or an extra Help action, or another round of nova damage when the fighter burns his Action Surge and goes to town. Given the choice between a few magic items in the party... or an extra level for each PC... or 1-2 more warm bodies... you always pick the warm bodies. Numbers count.
2) I intentionally split the hag encounters, and didn't allow the second pair to enter until the first was dead. Fighting all three at once would have been a guaranteed TPK against a 3rd level party with no items. Also, I intentionally spent the hags' opening round making their appearance, rather than moving and immediately casting a spell. This gave the PCs time to react.
3) I mixed in some claw attacks. If you just go with spells, the party is in real trouble. Frankly, unlimited
magic missiles is all they really need. Two hags will drop 21 average damage per round if they both cast MM; no attack roll, no save. That's brutal for a 3rd level party, even with healers.
4) Don't use
counterspell. That's a dick move on 3rd level parties, given a hag coven has slots to keep it up all day long (against the spells that matter, like
moonbeam). Also, be judicious with the offensive coven spells. The best attack is probably
lightning bolt every round until the party is a pile of ash, but I only chose to use one for scare effect. The
eyebite and
phantasmal killer spells are just as scary, but less likely to lead to a TPK.
5) I didn't coup fallen PCs, obviously. I have done this before, but the fight didn't warrant it. I spread damage around on the PCs that could absorb it, and carefully managed the perceived threat level by countering the cleric's/druid's healing by using
magic missile to keep them burning spellslots.
Could they have actually lost characters? Absolutely. If the druid had failed his second save against
phantasmal killer, he could have been back-doored (reduced to negative max hp; I would have needed 22 damage from 4d10, which is right on the average). At least one fallen character was "rolling on two death boxes" before he was healed. More PCs would have gone down if the cleric/druid hadn't been extremely careful with healing. And the party tactics were very good - good choice of spells, keeping the hags from meleeing with the casters, occasional use of special tactics like Help or Shove (to knock prone), quick recognition of which damage types were working (e.g. silver, radiant, force, etc).
The beauty of this encounter is that it also scales for higher level parties. By changing the actions I took above, I could have easily manipulated the encounter to be a deadly fight for a 7th level party. All the hags attack together. They cast in the same round as appearing. They actively use
counterspell and their best offensive coven spells. They focus fire on the healers first. That's part of the challenge (and fun) of being the DM. Manipulate the monsters' tactics to provide a credible threat (which occasionally kills PCs; let the dice roll how they may), but moderate it to the extent that you don't simply overwhelm them.