The Unseelie court in fairy lore consists of the "evil" creatures of feykind, such as hobgoblins, dark elves and bogles. They are typically the malevolent sorts who delight in tormenting sometimes murdering unwary travelers and make no distinction between good and bad character, unlike the Seelie Court who will typically only assault a person for an error or offense of some sort.
A Seelie Fairy may punish you for unknowingly stepping on a fairy ant hill, while an Unseelie Fairy may chase you down and beat you with chains because they happened to notice you walking at all.
However, if a mortal achieves some noteriety and respect, they may decide to make them a part of their court. Not exactly a cushy deal because that means either as a slave or a pet, depending on their whims.
(More details to follow as this court enters the story proper!)