|“|| Daylight... harnessed as a weapon!
Ultraviolet ammunition is a type of bullet used by Lycans to kill Vampires. They are hollow bullets filled with a special irradiated fluid that emits ultraviolet light. Selene takes a Desert Eagle loaded with the special bullets off a Lycan warrior, Trix, in the beginning of Underworld. Realizing that the bullets emitted UV radiation she brought them back to the mansion. When Selene shows Kahn the bullets, Kraven scoffs at the possibility of the Lycans developing such technology from scratch. However, Kahn theorizes that it was originally some sort of advanced tracer-round technology developed by a human military, which the Lycans then stole and adapted to their own uses, i.e. to emit specifically ultraviolet light.
The advent of Ultraviolet ammunition gives the Lycans a significant leg up in the arms race against the Vampires, equaling how the Vampires use silver ammunition against them. The Vampires (specifically Kahn) respond by making new advances in silver weapons, specifically bullets tipped with silver nitrate solution, which immediately disperse into the bloodstream.
Underworld: Evolution reveals that it was actually Andreas Tanis who designed the alterations to the tracer round technology to make them into UV ammunition. He manufactured and sold them to Lucian in exchange for Lycans to protect his home.