User blog comment:Dr.Who1997/Theory: Nate is a mole for Prometheus/@comment-24152851-20170202113051

How did Prometheus know about the Legends? Well if theories that he's Tommy Merlyn are correct, Malcolm could have told him at some point since he's busy fighting the Legends.

I can't say Nate being a Mole is impossible, but I don't think it's likely considering how many times he's put his hide on the line to save them. Arrowverse isn't that subtle with it's villains and usually drops some foreshadowing here and there.