Tech billionaire of facebook and napster, and his new project is curing cancer. What possessed you . Maybe it goes back to being a hacker. His plan of attack a revolutionary therapy thats saving lives like hers. I found out i was cancerfree. Reborn . Yes, reborn. I think this billionaire is on to something. But first, Richard Engel with an nbc news exclusive. I let my family down. Ive let my nation down. Why should i believe you . Hes the Young American who went from the ivy league to isis. He asked to help isis bring down planes. Why did he join . And why did he return . They just got more brutal and brutal. You saw heads on a stick . Yeah. Tonight, a dark journey inside isis. You can see madness in their eyes. That story right now on assignment. Good evening and welcome to on assignment, im lester holt. Tonight we take you into the very heart of the terrorist world and for the first time, an american who joined isis in syria has returned to reveal the inner workings of the terrorist