They hag leaves no trace while invisible. So you could roll a perception based on hearing the hag and know where she is but you still can’t see her. Disadvantage. Not to mention just because one party member can hear the hag doesn’t mean the rest of the party instantly knows where the hag is. There’s a later post I greatly agree with stating a hag with no minions is a very bad hag. They definitely can be deadly, by themselves they’re not.

Quote from Sninctbur >>

This monster looks really beefy to just be CR 3. Is there something I'm missing?

 Only one attack per turn.  Then again, it seems weird for a more magically-inclined monster to be super tanky.  Maybe improve her spellcasting a little bit, but tone down the hit points and AC.  

Can someone explain a bit more about how Invisible Passage works?  It talks about how it ends, but how can it begin?  Can she "cast" it multiple times?  Can she cast it in battle?  Like, is it more of an escape feature, battle feature, ambush feature?

Escape, Ambush, General trickery when combined with her mimicry and also false escapes.
Yes she can cast it muitiple times and yes it can be cast in combat.

If a player wanted a Green Hag Coven warlock patron, which existing patron fits best? Maybe reskin the water genie? Bottle is a cauldron? Or the newer Undead? Or is the Archfey the right choice?

Why do green hags know Draconic?

Quote from ClavacusVile >>

How does she have a 17 for natural armor, as much as a tortle? And only a 12 for dex? How?

 Natural armor means armor in addition to your dex mod

It's 3, I got the book in front of me rn

Quote from Sninctbur >>

This monster looks really beefy to just be CR 3. Is there something I'm missing?

She’s a hag. A lying snake who ruins peoples lives as a hobby. And now some naive DMs life is miserable after they destroy their first level party as she throws them across the room like a rag doll because they don’t check the stats. I would agree in most cases about how confusing CR is. Here, however, it is quite fitting.

Quote from Pancakeli >>

Quote from Zaboem >>

The Green Hag seems to have great stats and magic at a glance, especially for a CR 3 critter. Think about how it actually goes over in gameplay though. The hag relies far to heavily on deception, using either its Invisibility or Illusion. Players cry to roll their Perception Skills non-stop. They keep crying until someone makes the roll. It only takes one PC to make that Perception Roll, and then everybody attacks it. Hags have no ranged capabilities except Vicious Mockery and no spells that teleport them away to escape when they get in trouble. I've tried using hags as boss encounters, and none have yet to see one survive long enough to even get an attack off.

I do like the hags in terms of lore and their visual image, but as monsters, they are just easy and unsatisfying XP.

 
A hag caught in a fight by themselves has failed as a hag and is doomed to die, they're not built for killing stuff by themselves, and like most other low CR creatures, suck. If you want a hag to be scary in combat, it cannot be by itself. It might have familiars, beasts, and monstrosities that it's tamed, people they've trapped in magical deals (or otherwise willing to fight by the hag's side), or even be a part of a coven of other hags that are more powerful than it.

If you plan on having a hag by themselves, they'd likely have an escape route or two planned. They would recognize that the way they conduct themselves often angers others, and that there's little chance they could defeat an entire group of determined adventurers. Not every fight has to be to the death, and when one side can turn invisible at will and is clearly outmatched, that side would probably at least try leaving, and maybe coming back for vengeance later.

 I love having Green Hags as NPCs that manipulate and make deals with the player, but they don't work super well as a boss, especially because all they do to attack is scratch you with claws. As a DM I love to roleplay hags, they're amazing NPCs if you do them right, maybe give them a lair. As for fighting though, I'd just give them some extra spells and maybe let them summon the zombies of their past victims to make the fight more interesting and difficult.

Frog lady is mean

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