The Beastlords were a group of cannibalistic tribals who underwent a mutation which gave them psychic control over various creatures. They were fond of poison, which can cause problems for even the most stalwart of bands. In 2197, their leader was Emperor Daarr.
Around 2197, the township of Quincy petitioned the Brotherhood of Steel for protection from a large group of the Beastlords. These Beastlords, under the leadership of Duff the beast master and an unnamed Beastlord commander, took over certain parts of Quincy and were using hostages to ensure they have control over the rest of the town. Among those captured was Hillary Eastwood, the mayor. A Brotherhood squad was sent to rescue the mayor and assist in dealing with the Beastlords.
After rescuing Quincy, the same squad was sent to the Beastlords' lair in Mardin, in order to eliminate the psychic cannibals to delay their expansion, and possibly learn how to control the beasts themselves. While the tribals were eliminated, the latter was not achieved, since that would require undergoing the same mutation as the Beastlords had done.