The Khans are the most dangerous of the three raider groups that came from Vault 15. They live the lifestyles of Mongol warriors, raiding towns, burning what they cannot take and capturing the survivors for use as slaves
In 2161 The Khans set up camp southeast of Shady Sands and attacked the (then small) vilage on a regular basis. In one of these raids Tandi was captured by them and held captive by Garl (the leader at the time). The Vault Dweller (at request of the town) then traveled to there camp to rescue Tandi and killed all but one of The Khans.
Darion, who was the only survivor of the attack by The Vault Dweller became obsessed with getting revenge on Tandi and NCR. He was riddled with survivor guilt because he had lived, while the rest of his gang had been slaughtered. He rebuilt The Khans under the name of The New Khans. In an ironic twist of fate, Darion was killed in 2242 by the Chosen One, the grandchild of the Vault Dweller who had killed his companions 80 years earlier.
After The Khans were nearly destroyed by the NCR at Bitter Springs, the remnants were led by Papa Khan who took them to the Mojave Wasteland and formed The Great Khans.