Now he's paranoid. He believes that Tandi is out to get him. He's obsessed with revenge, and he wants Tandi dead. He's riddled with guilt 'cause he hid and lived, rather than dying with his gang... and the list goes on and on.
Coming from a Khan background, Darion is the sole survivor of the Vault Dweller's massacre when the latter came to the rescue of Aradesh's daughter Tandi, a girl the Khans kidnapped. Darion hid during the fight, an experience that haunts him ever since (Survivor's complex).
Guilt-ridden, he set out to rebuild the might of the Khans, and over the eight decades that followed he established and ran the New Khans gang. His experiences made him paranoid and obsessive over the years. They drove him to seeking revenge on the Vault Dweller and Tandi, now president of the NCR.
As part of his plan, his men occupied Vault 15, blocking NCR's access to their place of origin, using squatters as cover, while a spy was planted inside the NCR congress. Growing quite old and fearing death might find him too soon, he had a doctor kidnapped, in order to make sure he was still around to carry out his revenge personally.
In a twist of fate he eventually died on behalf of the squatters by the Chosen One, grandchild of the man who had wiped out the old Khans 80 years prior. With Darion gone, the New Khans' hold on Vault 15 quickly broke, and the gang was hunted to near extinction by the NCR for the second time.