I'm pretty sure that there are two sides to the Wall of Skulls, with the Ghoulish Trio on the side where you start and the Mushroom Forest on the other side. Climbing the wall will show you what's on the side you currently occupy. The two encounters are mutually exclusive—you won't get the Mushroom Forest until you break through the wall, and then you won't get the Ghoulish Trio again until you break through back to the other side.
Fires Of The Black Book / Discussion
This is the discussion related to the wiki page Fires Of The Black Book.