Sorcerers and Spelunkers
Zuggtmoy is an abyssal lord.
When the party brought Stool home to their mushroom people at Neverlight Grove, the residents were undergoing a religious revolution. Soverign Bassidia indicated many mushrooms were covered in strange spores and were behaving oddly. They were preaching of the Day of Joining and were eager for the party to visit the garden of welcome.
As it turned out, the garden of welcome was a monstrous collection of decaying humanoids being cultivated in fungus for some terrible purpose. An ambiguous marriage that would bring forth the Lady of Decay, Zuggtmoy.
The party fled Neverlight Grove with the mushrooms loyal to Sovereign Bassidia.