Nightstone Unlimited is the corporation founded by Demona and Thailog while they were in Paris, combining their accumulated wealth and using their "human aliases" of Dominique Destine and Alexander Thailog.
The corporation focuses strongly on genetic research, and in particular, created the CV-1000 Carrier Virus in order for Demona to carry out her failed Operation Clean Slate plan. ("Hunter's Moon" Part Two) The Clones were also created at the Nightstone Unlimited building in Manhattan. ("The Reckoning") Alongside genetic research, Nightstone Unlimited has its own coffee franchise in London and perhaps elsewhere. ("Rock & Roll")
Demona/Dominique Destine has seemingly abandoned it following the events of the Hunter's Moon, and Alexander Thailog now operates it solo (by speaker-phone, memo and e-mail to keep his true nature a secret).  Sevarius has often been hired by Nightstone to work on various projects.
Demona came up with the name "Nightstone Unlimited" and Thailog liked the sound of it. ("Sanctuary")