Zuggtmoy is the Demon Queen of Fungi. She shares a layer of the Abyss with Jubilex, the Faceless Lord of Slimes and Oozes. She built the Temple of Elemental Evil with her ally (and possible lover) Iuz.
While she's not my favorite demon lord, I've always had a soft spot for her. Though she's chaotic and evil, her domain includes natural elements. Introducing a group of corrupt druids that worship her could be interesting.
Another reason I love Zuggtmoy is that she gives me the perfect excuse to use myconids. I mentioned my love for the shrooms when I did the 30 Day D&D Challenge.
If you'd like to include her into a campaign, you can find some useful inspiration from Magic the Gathering's Golgari Swarm. This green/black guild from the Ravnica and Return to Ravnica blocks was made up of necromancer/druids that practiced rot farming and saw life and death as natural. If you'd like to use Zuggtmoy as a dark nature patron, you could take this route.
I didn't want to stat Zuggtmoy herself, so I decided to stat one of her assassins on the prime material plane, the evil myconid known as Deathcap.
Deathcap, Zuggtmoy's Rotting Blade
Little is known about the myconid known as Deathcap. He speaks no languages, but understands all, thanks to the blessing of his goddess. An outcast from his tribe, the mushroom man travels the dark paths, spreading the gospel of his Zuggtmoy, one painful death at a time.
Armor Class: 4
Hit Dice: 3
Move: 120' (40')
Save As: Fighter 3
Deathcap is immune to poison, sleep, and paralysis.
Deathcap's natural form allows him to blend in to caverns and areas of heavy vegetation. If standing perfectly still he is considered to be invisible.
Once per day Deathcap can make a spore attack. This attack automatically hits all living creatures in a 20 foot radius around him. Each individual must make a Saving Throw vs Death-Ray/Poison. If they fail this saving throw, they are affected by one of the following effects (determined by rolling a d6, caster level is considered to be three): 1 - Cause Fear 2 -Cause Light Wounds 3 -Blight 4 - Hold Person - 5 - Cause Disease 6 - Confusion
He wields the magic longsword +1, Canker, gifted to him by Zuggtmoy herself. The blade is rusty and fungus covered, but sharper than steel. It reeks of mildew and old blood. Those hit by the blade must make a Saving Throw vs Death-Ray/Poison or be infested with fungi and molds. This infection causes the victim to lose 1-4 points of charisma until the infestation has been cleared via a Cure Disease spell.