The Unseelie Court's the antithesis of everything normally thought of the faeries of the Seelie Court. Whereas the Seelie Court is filled with laughter, friendship, and loveliness, the Unseelie realm is filled with hatred, enslavement, and death.
Deep within Phlegethon and far from any usual routes of travel, the Unseelie Court is centered within an immense cavern hundreds of feet high and filled with stupendous stalactites and stalagmites of blackest stone. The center of this cavern is dominated by a single mountainous stalagmite, atop which sits the black onyx throne of the Queen of Air and Darkness. Resting on this throne is the ten-faceted Black Diamond, the gem which legend says corrupted the sister of Titania and set her on a course to destroy the Seelie Court. The Queen of Air and Darkness no longer manifests herself in a physical body. Rather, her presence fills the entire cavern, but it emanates from a cloud of blackness that hovers about the gem and throne, flickering with continual lightnings that reflect from a myriad flecks of crystal in the cavern's walls and columns.
All around the central stalagmite, undead and petitioners toil away at their appointed tasks. Some minister to corrupt faeries recently returned from missions for the queen. Some provision agents soon to leave. Others work to create new items of magic to confound the queen's enemies. More than one lich has fallen prey to the dark queen and found its wizardly powers enslaved to the evil faerie queen's will.
Yeth hounds and hell hounds are favorite servants of the Queen of Air and Darkness, and scores of them prowl the outlying areas of her realm, hunting for intruders. Especially faithful faerie agents are sometimes awarded one or more of the evil beasts to aid them in their missions.
There are no towns or settlements within this realm. Its dark queen prefers to keep all her subjects near her throne when they aren't afield specifically on her missions. And other than those assignments, they have no truck with humans the way that some faeries of the Seelie Court do, so there's no settlements of other races nearby. In other words, the Unseelie Court isn't a fellowship or culture, but rather a collection of beings in bondage to one evil mind.
There's only one driving force in this realm, and that's bringing the Seelie Court and all its subjects under the sway of the dark queen's corruption. No other business is done; no other thoughts are condoned. Sods who stumble into this realm are summarily judged. If they seem a potentially useful tool against the Seelie Court, the Queen of Air and Darkness enslaves their wills and adds them to her army of agents. If they prove unfit or intractable, they are destroyed - or worse, saved for use in some foul magical experiment.