Each time the Cheifton UFthak attacks, a resource is placed on him. Once he is defeated, do the Hero's that defeat him, keep the resources or are they returned to the main resource pile? I assumed they were "won", just not sure if this is true.

They are discarded. The only purpose they serve is as trackers to determine what Ufthak's attack strength is. They are not won or transferable.

Hmmmm...that would be a potentially awesome ability to give some hard to defeat enemy though...Like a "Moria Metal Worker" who comes into play with some amount of resource tokens...once you defeat him you take the resources and divide them how you want to your heroes. ???