The Scroll of Secrets will allow you to see secret passages (hidden doors between rooms) on the map screen. It can be found under the bottom room in the fifth column on the map in the Chalice of the Gods dungeon. The arrow underneath the room indicates where the scroll is actually located.
To get to the scroll, you need to fall from the room above it, which looks like this:
To get past the snakes, you need to go to the room to the left of this one. When you're in that room, grab the bullets, then jump over the snake and go to the bottom left corner.
You need to fire two bullets from this spot toward the right side of the screen. The bullets will travel off the right side of the screen and you'll hear them hit the snakes.
Once you've done that, go back to the room on the right and fall through the gap in the bottom right corner. You'll reach the room seen below. Touch the scroll and the achievement will unlock.