Then where is it!?

It is probably a multi-chapter quest and continues past the early access area.

I found several more clues. Can anyone confirm it's not yet in the early access?

Talk to Priestess Gut, follow her to her room. There's a door to the right. Go in there, go past the ogre, you'll find a door that leads to a dark room with a spotlight. Then make the floor match the wall .

You don't have to talk to Priestess Gutt, you can jump over to her room where the wall ends to the right side of that door and you can ender her inner chambers. There's a big guard patrolling the area, you can kill her, sneak past, or convince her you're leaving. Go through and there's a puzzle, the instructions to solve it are in a book next to Priestess Gut's bed. You can either solve the puzzle or if you succeed a perception check there's a lever you find on the right hand side of the room from the doorway you entered through. Lockpick it. A hidden door opens whether you use the lever or the puzzle. Using the puzzle gives you inspiration if you're a sage or have Gale in your party. You go through the doorway that just opened and there's a ladder. You don't need to jump or use featherfall. You only need that for the entrance to the underdark that's under the big spider matriarch.