The Dark Army of Night was formed by Lord Vanrak Moonstar, a minor Waterdeep noble, after his father was killed by a mysterious, uncurable wasting disease that was possibly brought back with him after Vanrak returned from an expedition to the Black Jungles of Tashalar. Vanrak, his body and mind weakened by his arduous travels, was overcome with grief and blamed the priests of Selûne from the High House of Stars for his father's death. So engrossed in his grief, Vanrak was an easy convert to Shar's worship.
Vanrak assumed leadership of the family and quietly started converting family members and servants to his new faith. 26 years later he openly declared his faith and organized a small mercenary army, led by Sharran preists which he used to terrorize Waterdeep's harbour.
Vanrak's sister, Lady Alathene Moonstar, in 1130 DR took refuge, with the rest of her Selûne-worshiping family in the High House of Stars and petitioned the courts to strip her brother of his title and banish him, providing evidence of numerous heinous crimes. The Watch came to apprehend him but unbeknownst to them, Vanrak had caught wind of their plan and taken his followers through a secret portal to Undermountain.
After 41 years, Vanrak and his followers had conquered a sizeable portion of the part of Undermountain that they lived in. It was called Vanrakdoom after it's master and eventually they would hold the entire sublevel. The Dark Army used it to attack Selûnite targets on the surface whilst Vanrak himself consolidated his power base and pursued his personal goal of immortality.
11 years after that, in 1182 DR the Dark Army scored its greatest victory to date, managing to infiltrate the High House of Stars, kill it's inhabitants and burn it to the ground with darkfire. The loot that they plundered was enough to fund Vanrak's ascension to a Death Knight.
The Dark Army hasn't been nearly as successful since. The High House of Stars was rebuilt as the House of the Moon (which they have failed to resign to the same fate) and all of their major plots against Selûne's faithful have been foiled. However, Vanrakdoom is safe enough and their ranks have swelled, most recently from about 80 Krinth refugees from Thultanthar.
Apart from the Krinth, the Dark Army of the Night consists of many clerics (as Vanrakdoom is mostly just a giant temple to Shar), rogues, sorcerers and their fighter guards. The elite members are usually members of prestige classes that revolve around shadow such as shadowdancers, nightcloaks and shadow adepts. The Army also has control of many monsters native to or affiliated with the Shadow Plane like nightwalkers, nightshades and a pack of 12 shadow mastiffs led by a shadurakul.