There was a relatively small number of them left by that time and Fain had sent them to wait in the forerested hills above Falme (outside a stedding, IRRC). They probably ate the remaining DF then started to roam around. There were Domani armies and Taraboners on Almoth and Toman Head, so they likely got killed eventually.

It sounds like Fain did not return to them but got interested in CoL/Questionners on Almoth. Mordreth (who doesn't know them) must have sensed their good potential for the "Shadar Logoth" philosophy and Fain made his way to Amador to try to use Niall. I guess he expected Niall would be "rabid" and easy to use, like Carridin's men on Almoth.