You can certainly find a YouTube video showing you exactly where this is if my description sucks.
No - easy peasy. I found it quite quickly.
Had a hunch already.
Because when in battle and when the AI's turn comes, the battlecam was zooming EXACTLY to this place anyways for no reason.
The whatever coding responsible somehow thinks this area needs to be focussed during one of the AI turns.
I just thought this is just a typical Early Access bug.
Yeah this wall is not on the path when someone walks by normally.
Found it pretty easily when I hugged the wall though as I proceeded.
Well that was exciting.
Thank you very much!

The Zentharim are a cocky Mafia that's for sure but I must admit they have balls.
Finding this elevator reminded me how daring they are to build a hidden storage place in the Underdark of all places.
Found their place in the Underdark normally of course but somehow it felt like the Zentharim we encounter in the first Act may not be that related to this place after all.
As it seemed to be in a deadend - I kinda figured it might be a place long forgotten for about 10 to 50 years or so.
Is this the door from the area with masked guys to another area with poison clouds?
Just how do you end up there with hag already dead? Unless you killed her right there in the house before she managed to vanish?
The MLG methodIn this game, positioning is everything.
If you plan to attack this green D&D Babayaga in her hut, you can place your group on higher levels in her house beforehand.
Attack her before or after the dialogue.
She is not becoming hostile immediately after you reject her offer.
Ethel will teleport this pregnant Human Woman away - which is good to avoid her becoming collateral.
Wait until Ethel stands in a favourable position to just nuke her with your strongest ranged attacks.
But as usual,
"Smokepowder C4 barrels" are an all time favourite for hitting hard.
I prefer to lit an enemy on fire first and throw it right on them with another character.
Very sweet hitchance for the big explosion damage ( feels like 100% ) and Ethel teleports only after 3 rounds of combat in her hut.
If you are experienced with positioning and how to deal damage in this game, she might not even survive round 1.
Alternate - if Ethel gets awayShe fights much more prepared against us around the cage area, with annoying illusions and super overpowered range for all of her attacks.
I suggest you shoot a single Magic Missle on every "Shadowclone" she summons of herself to stop it from making the battle unnecessarily hard.
Her real body is invisible as long as her clones exist.
Try to pop at least two of those damn clones with one spell usage.
Give other partymembers Scrolls of Magic Missles if you have any so they can help.
During Ethels round her stupid subtitled laughter reveals where her real body stands possibly.
An AoE attack can help you to force her out of invisibility and maybe finish her off without the need to kill all of her clones.
But the clones are a hassle anyways and if her real body has still much HP, defeat her clones as fast as possible.
Once you have killed the monstrous Fae Granny, you can equip the Mask anyways.
Backtrack the way up and keep to the left while you do so.
At one point there will be a small rift you must jump over.
An illusionary door awaits right afterwards. Equip mask -> move the camera sideways on the other side and click on the ground behind the door. You pass most easily.
€dit:
Do not venture into the Underdark with Ethels disgusting Debuff. Sleep at camp first to reset it.
There is a Landshark creature down there with an already powerful acid attack.
It becomes a oneshot most easily if you still have Ethels debuff.