This page offers a guide on where to locate and how to complete the Fly Agaric Mystery in Grantebridgescire.
Fly Agaric Trip
This Fly Agaric Mystery is located southeast of the Isle of Ely Monastery. You’ll first need to eat the mushrooms before you can access the puzzle. When you’re hallucinating, you’ll be able to see red flames in the braziers around you. To complete this puzzle, you’ll need to turn some of the brazier flames blue by interacting with them.
It is a little tricky to figure out which ones need to be blue, but you’ll need to look for the stone gates that overlook specific braziers. The brazier to the North, West and Southeast need to be turned blue. When you have turned the three correct braziers blue, a gate will appear and you can go through it to complete the Mystery.