Vigilante Nocturna
The Nightwatcher is a remnant of the Old Magic that was the prevalent religion across Roshar before the rise of Vorinism. The Nightwatcher would bless those who sought her out with one blessing of her choosing. In exchange for this blessing, she also bestows a single curse, although this curse very rarely has any obvious relation to the blessing that was gained. Because of the Nightwatcher's ethereal nature and ties to a single location (the Valley), she is considered by many to be a powerful spren.
Blessing
Anyone is allowed to seek a single boon from the Nightwatcher. To do this, the supplicant must first travel to the Valley in the west, where the Nightwatcher resides. After finding the Nightwatcher, the supplicant may request a boon. There are no clear restrictions on what the Nightwatcher has the power to grant--like most things regarding the Old Magic, the few known facts are surrounded by a web of myths and legends. The precise phrasing of the request does not appear to make a difference, however, since the Nightwatcher bestows a boon that she feels the supplicant deserves. In most cases, the continued pain of the curse appears to outweigh the blessing of the boon, even if the supplicant receives the boon they desired. A notable exception was Av's father, who received enough cloth to keep his family from starving during a famine. His curse, seeing the world upside down, apparently stopped troubling him after he acclimated to the situation.
Curse
Contrary to popular belief, it is not possible to avoid the curse by carefully phrasing a request--or even to predict the curse that the Nightwatcher will bestow. Interestingly, it does not appear that balance is carefully maintained through an individual blessing and curse--often, highly visible curses are the result of relatively minor boons. The curses can be quite thorough, however--Dalinar lost all memory of his wife, and cannot even focus when her name is mentioned in conversation. In another case, a man lost all feeling in both of his hands until the day he died. Whether these curses follow some cosmic justice, or are merely the result of a capricious spren, is unknown, as is the true nature of the Nightwatcher.