From Guild Wars 2 Wiki
Imbued Mushrooms are mushrooms found in Slothasor's Den in Salvation Pass. Eating this mushroom makes the player undergo a Magic Transformation, turning them into a Slubling that is hostile to fellow players whose purpose is to eat the purple Poison Mushrooms to clear a path of unpoisoned ground around the room. Imbued Mushrooms slowly spawn in four fixed locations, rotating counter-clockwise from where the first mushroom is at the entrance to the den.
Having a player eat the first Imbued Mushroom is required to start the the Slothasor boss fight.