The Underdark

The Underdark is a vast, dangerous, and perpetually dark subterranean realm lying deep beneath the surface of the Sword Coast. It is a world of ancient shadows, treacherous caverns, and home to malevolent civilizations and monstrous entities. For surface dwellers, it is a place of legend and terror, representing the source of many threats that have historically plagued the mortal kingdoms above.

Subterranean Inhabitants and Threats

Historically, the Underdark has been the breeding ground for “worse things that crawled from below the earth.” During the decline of the ancient Pact of Phandelver, these subterranean forces surged upward alongside orc hordes and goblin banners, contributing directly to the collapse of kingdoms like Highthorn and the abandonment of Axeholm.

Among its most notorious denizens are the Drow (dark elves) and Driders—drow who have been transformed into horrific half-spider, half-elf abominations. The Underdark is also home to bioluminescent ecosystems, including massive colonies of Myconids (sentient fungi) and glowing mushroom networks that run deep into subterranean passages bordering locations like Wave Echo Cave and Gnomengarde. Within the deep caverns, Myconids share a symbiotic relationship with the nearby gnomes. However, these peaceful creatures are frequently exploited; goblin forces serving the Drider Nezznar have been known to slaughter stray Myconids to make stew, boarding up the cavern passages afterward.

Following the defeat and death of Nezznar the Spider—who was slain when the half-elf rogue Er’evyn shot him through the eyes—the Underdark forces in Wave Echo Cave collapsed. The remaining goblins and bugbears who fled were consumed by a vengeful mushroom patch. In the aftermath of this conflict, the subterranean mycelium networks began to flourish, feeding on the remaining spider and orc corpses and clearing out residual undead.

Additionally, the deep territory is home to Phase Spiders, formidable arachnids capable of turning invisible and teleporting between planes. These creatures were frequently utilized by subterranean factions as stealthy couriers to convey secret missives, such as those addressed to the dragon “Wyrmspeaker,” to the surface world.

Factional Dominance and the Black Network

The Underdark is not merely a wild cavern system; it is a region of complex political intrigue and criminal expansion.

• The Zhentarim Schism: The ancient Black Network of spies, thieves, and smugglers maintains a major presence within the deep territory. However, a deep schism divides the organization. One faction, historically overseen by the ruthless Underdark division leader Nezznar the Spider, represents a highly violent and expansionist arm. Nezznar’s operations ran parallel to other regional Zhentarim leaders, but he harbored immense disdain for those based in the Lord’s Alliance Great Cities—such as Lucian Nightshade of Neverwinter, Jamra Gleamsilver of Waterdeep, and Halia Thorton of Phandalin—deeming them “weak” for living among civilized folk. This internal conflict culminated in the murder of Phandalin spymistress Halia Thorton by Nezznar’s forces before his own demise.
• The Pact with the Dragon Cultists: Before he was slain, Nezznar sought to establish a grand geopolitical split between the surface world and the depths. He entered into a secret alliance with dragon-aligned factions (referred to as “the Dragon ones”), negotiating directly with an emissary known as the Jade Wyrmspeaker. In exchange for delivering the magical item bounty of the legendary Forge of Spells to the Cult of the Dragon, Nezznar was promised absolute control over the Underdark and all of the Forge’s remaining riches. This alliance was a vital bridge for a massive two-pronged operation: while the Cult worked to resurrect the green dragon Villathas, Nezznar worked to subjugate the subterranean depths. However, this dark alliance was strained by the Cult’s incompetence, notably when their captive white dragon, Cryovain, escaped their grasp and distracted the surface forces of Phandalin.

Religious Tensions

Spiritual warfare runs rampant within the Underdark, particularly centering on the worship of Lolth, the Spider Queen. Subterranean agents of Lolth actively work to defile altars of surface deities—such as the All-Hammer, Moradin—converting them into shrines dedicated to the Spider Queen. Nezznar defiled the ancient altar of Moradin in Wave Echo Cave, draping it in thick webs and spider eggs before they were eventually put to the torch by the fairy Nalini.

Nezznar’s devotion to Lolth was tied to his grander ambitions. He believed his alliance with the dragon cultists and the conquest of the Forge of Spells would secure Lolth’s absolute dominion over the subterranean world.