There are actually three skills there, I unlocked them after the fight against the spear wielding monk/nun lady in front of the temple that iirc you reach from the branching path in the forest.
to put it simple its a skill that can't be unlocked by point but u need to do specific thing to unlock it. just like certain item that can be optainable in other game when you progress the main story, to make people can't reach beyond things to do if they refuse to do the story.
same question here, but also: Where can i put black rusty sword from bones mine? Npcs (miko and spear lady) ignores it. Broken stone with glyph on it (on forest temple map) doesnt do shit either
Now that you've seen the first picture, go back to the shrine in the first area and talk with the maiden, you'll get a new passive skill that prevents poison; that will let you get to a new area, at the end of which is a thing that will make the second picture's barrier disappear. The third picture is just the end of the level for now, it's basically an "under construction" sign.
I just finished the demon pool but the purple passage still hasn't unlocked
There's a statue at the end, just past the boss; there's something you can do to said statue that removes the barrier. It'll be pretty obvious when you find the right thing to do.
