"They are sent from Mordor," he says flatly, "and they fight with terror as well as blades."

Then, remembering that he's speaking to one of the few people in Middle-Earth who's never heard of Mordor, he adds, "From the Dark Lord Sauron, to kill us or worse."

Gord has no more idea who the Dark Lord Sauron is than about Mordor. As far as epithets go, and by demonic standards, Dark Lord is practically wholesome.

They're not telling him why these Black Riders are after them, or asking for his help, or even saying they don't want to fight them, per se. Fair enough. He does have a selfish reason not to want to leave, because these people are his only source of information right now, but if they're attacked while he's there it will be much harder to convince the attackers that he's a neutral party.

...wait. "Merry. Translate what I said." He picked a good target for Share Language, because Merry speaks unguardedly, but clearly not the best one for coordination during a possible fight.