Did everyone stumble upon her like me doing the Hidden Path quest? Or is there a quest lead somehow suggesting to look for her. I had heard of her just as a super boss (the name) but had just heard the name thrown around and didn’t know anything about her. It reminded me of Toxeus from Titan Quest a little bit in that I was just like whoa so that’s who killed me haha.
No quest involved, I just found her by meticulously probing every corner of the room, like I do with every other room in this game, because I’m obsessed with map completion.
I heard her name, googled where she was…And that’s also how I found the Hidden Path, actually.
I admit, on my first character, I looked everywhere, I made sure to reveal every picky little corner of every map… I even found the start of the Path before googling…But I never remembered to go back once I actually had Dynamite. Same with the Flooded Passage area, actually.
I found her during testing the Hidden Path. If I recall correctly, all Zantai initially told us for the quest line was that we could find the start of it in Lower Crossing, that was the only tip.
As for the Mad Queen I found her entrance rather easily on first exploration of the cave. It was much more obvious that first time and because us testers had found it so quickly and easily they made adjustments so it wouldn’t stand out quite so much. Even the “door” to her hideaway was a bit wonky and your character could partially walk into it, which I commented on and they fixed.
That was how I “stumbled” upon it.
The entirety of the questline took me between an hour or two max to solve and complete but I liked it and thought it was fun and enjoyed the lore it added to the game.
Literally just happened to mouse over the rock that opens the door, I called my friends over before opening it and we all stacked up on the door and went in. Chaos and death followed. 
Found her before i saw spoilers on forums. I noticed the entrance was similar to Hidden Path so i moved cursor here and there and found it
Frankly i was disappointed on how quickly the spoilers for the quest made to the forums
To be honest the hints were fairly obvious, I solved it myself in about 20 minutes.
Did die horribly at the claws of the queen that first time :o
I came to this board one day and saw everyone is talking about some Mad Queen 
. Had to Google it to catch up
I didn’t look well enough to find her. I think I asked powbam on a thread somewhere lol.
Mouseovered the rock by coincidence, while doing “hidden walls” check on every inch of every new wall I come across. Killed her easily on elite. Came to forums, noticed everyone was talking about her as she is some kind of ultraboss. Didn’t understood. Died hard to her on Ultimate later. Amended my ways 
^
Having the same mental condition
The Hidden Path’s location’s weren’t that hard to find for me, I remember only struggling a bit with the Bysmiel one as the “fortress ramparts” one was a bit away from the actual fortresses.
Though I’m sure that the start of the questline is not easy to find. We just got lucky that Zentai mentioned to start our adventure at Lower Crossing.
At least it’s not as bad as Symphony of the Night, I don’t think I ever got 200.6% map completion or whatever the hell it was. 
Also if you didn’t know, once you fully uncover the map with one character you can copy a file from that character’s save directory (I think it’s map.fow) to your other characters and not have to do it again.