To earn the Shroomtender Breeches, you need to complete the Bane of Heralds achievement.
This requires you to defeat three bosses in Bastion Nymic - the Nymic Refuter, Nymic Defier and Nymic Purifier.
You can access Bastion Nymic once per day on each character. To do this, you'll need to speak to Ordinator Tandesa to pick up the Bastion Nymic Daily Quest. You then need to collect 5 Daedric Ichor by finding and defeating Herald's Seekers. If you are grouped with friends then you can complete this objective more quickly, since each Daedric Ichor collected by someone in your group also counts towards your count.
After you have collected the required Daedric Ichor, you can use the portal to enter Bastion Nymic. This is a new type of instanced world event, designed for groups of four players, but if you're skilled at the game you may be able to do it with just one or two friends, or even solo.
Once inside Bastion Nymic you are free to explore as much as you like and complete as much or as little of the bosses and puzzles as you want before leaving. However once you leave you will be barred from entering again until the following day, where you'll need to redo the daily quest to gain access.
