The Black Widow was a Bio Organic Weapon (B.O.W.) produced by Umbrella as part of their development program into Arachnid-based weaponry.
The Black Widows were being developed by Umbrella by the end of 1998; the company had plans to distribute them to a number of their facilities via the Antarctic Base.
During the Antarctic outbreak of the t-Virus, the Black Widows being transported took advantage of the emergency to break out of their crates and settle around the facility.
Like many spiders, the Black Widow possessed four pairs of legs attached to its cephalothorax, connected to the opisthosoma (abdomen) by a thin section. The Black Widow's appearance was disambiguated from other spider-based B.O.W.s by its complete lack of body hair. Like other spiders, the Black Widows would release little spiders from their abdomen if killed.
One of these spiders, like the Black Tiger, reacted differently to the t-Virus than others of its kind and, again like the Black Tiger, reacted by growing significantly larger.