|The following is based on Fallout Tactics and some details might contradict canon.|
Mardin is an underground cave labyrinth northwest of St. Louis, within which the lair of the Beastlords was located. The Beastlords possessed an inherent telepathic suggestion which allowed them to control various animals. It was acquired through prolonged exposure to an unknown radiation present within Mardin's underground caves. According to the Brotherhood scribes, this unique mutation only manifests in humans that have been exposed for at least twenty years. Around 2197, a Brotherhood of Steel squad was sent to Mardin to eliminate the Beastlords.
They attacked, killing many beastlords and their pets in the process. It was the Warrior who led the team, and when they advanced to the main room of Mardin, they finally found Daarr, the Beastlord emperor, and killed him either by their own force or by liberating Mother (the deathclaw matriarch), who was the source of Beastlord's control over deathclaws.
The mission in Mardin is to infiltrate the Beastlord's lair and kill their leader, as well as discovering how they are able to control the deathclaws.
Mardin appears only in Fallout Tactics.