It's about three years after they begin their attempt to integrate themselves with the world of humans that they learn humans are not alone.
They run across it quite by accident. Fëanáro is in Cape Canaveral, watching the launch of a research satellite and the maiden flight of the Delta 0100 rocket system. The launch is successful. He attends a party afterwards, and escapes because it's too loud, and hangs out on the rooftop reading minds and humming to himself and staring intently at the stunningly beautiful embroidery on the inside of his sleeve. There are a lot of minds to read so it takes him a second to notice that one of them is being murdered.
He stops humming and now that he's listening for it can hear muffled screams. It takes another second to find the right mind, and then to jump from the victim to the attacker, and then to realize -
Human minds are not all the same. It's hard to tell if they vary more than Elven minds or if it's just that Elves whose minds were so far outside normal variance would choose to obscure this. Probably they vary more, though - humans do, in most respects.
This mind is not human.
He does not intervene in the murder. He would have, once, but his grip on the world is far more fragile than it once was, and he's weaker, slower, unarmed, blocks away. And he doesn't have enough information. And Moryo would say that his comparative advantage continues not to be in hand-to-hand combat even if there happens to be some going on nearby.
And they die anyway.
He stares intently at the inside of his shirt and he concentrates on the collapsing thoughts of the dying man and he toys with Quenya words for the new species. Vampire, the dying man had thought. From the French vampyre or the German Vampir, presumably, those themselves borrowed from the Slavic languages, all of which had it: Bulgarian, Macedonian, Bosnian and Croatian all as vampir, Czech and Slovak upír, Polish wąpierz and upiór, Ukrainian and Belarusian upyr. The presumed proto-Slavic root was ǫpyrь, or maybe ǫpirь. He'd need to look it up, past that. It might make more sense to start with the Quenya word for shapechanging servants of the Enemy, though this creature didn't seem to serve anything beyond its own reckless hunger and glee at the deception that had enabled it to feed.
Mandulómi, maybe. Or Nuruhuine. He said both aloud.
He ventured back down into the party, plucked a carving knife off a serving tray, and walked to his hotel. He called his children. "There's something here we need to kill," he said. "There may be many of them."