The Lycan Den is a location seen in Underworld and Underworld: Evolution. It is the home of Lucian and his army of Lycans. Its location is never specified, except that it appears to be underground, underneath a city building. Here, the Lycans capture descendants of Alexander Corvinus and use their blood with that of Lycans to try and create a successful fusion of the bloodlines, in order to create the first Hybrid of Vampires and Lycans.
While its origins are unknown, it can be assumed the Lycans created the base after Lucian faked his death, burning down their old castle lair and making a deal with the vampire Kraven to help him fake his death in the process. Kraven gained personal glory in return, claiming to have killed Lucian and removing a piece of his skin with Viktor's brand on it. Kraven then repeatedly visited the Lycan Den after that to meet Lucian, although this often resulted in Kraven ranting.
With the assistance of the Lycan scientist Singe, Lucian became determined to create a successful fusion between the bloodlines, after his lover Sonja was killed by Viktor for carrying the first but unborn Vampire-Lycan hybrid. The Lycans began kidnapping descendants of Alexander Corvinus, who potentially possessed the Corvinus Strain, the key to achieving the fusion. However, all of the attempts failed, until the Lycans began tracking Michael Corvin. This attracted the attention of Death Dealer Selene, the adopted daughter of Viktor. Michael was made a Lycan by Lucian, and a sample of his was blood obtained, which carried the Strain. Viktor's early awakening by Selene, and Singe's capture as well, led to the Vampires attacking the hideout.
Kraven went to Lucian for safety, but then turned on him and mortally wounded him with silver nitrate bullets. Viktor led his Death Dealers into the hideout and a vicious battle between the two species occurred, Raze, Kahn and Soren all dying in the process. Lucian survived the bullet, convincing Selene to bite the mortally wounded Michael, who had been previously captured by the Lycans. Selene bit Michael, creating the first Hybrid. Lucian was killed by Kraven, but died with his goal fulfilled, avenging his murdered family. Kraven fled, shortly before Viktor arrived and fought Michael in his new Hybrid form. The battle ended with Selene slaying Viktor, and then fleeing with Michael. While at least two Lycans survived the battle, they left Selene and Michael alone and let them go.
The lair was likely abandoned after the battle, although the Cleaners arrived after to retrieve the bodies of Viktor and Lucian.