The day Sauron dragged himself back into corporeality and discovered Elrond and Celebrian had kids while he was MIA was one of the worst days of his life.
He’s not scared of Galadriel is a mantra he mutters to himself sometimes when things feel tough. She’s just… a lot. And Elrond has been a thorn in his side for ages, with his stupid Numenor connections and tendency to show up with an army at exactly the wrong time. Sure, he doesn’t seem to take after Luthien much, but Luthien didn’t seem to take after Luthien much until one day she showed up out of nowhere and just started destroying evil fortresses and ripping people’s throats out.
Now these losers are just throwing their genetic information together and seeing what pops out? Incredibly rude. The chances of a Luthien 2.0 (With Even More Menacing Prophesy) is not 0.
Fortunately news trickles out of Rivendell that the twins are just very intense jocks, but the best intelligence he can find on Arwen is “grandma’s favorite” and “kind of quiet” and that concerns him.
*gleeful cackling*
Now I’m imagining Sauron finding out about Aragorn and Arwen, and just freaking out. “And now the Concerning One is shacking up with Isildur’s Heir? Like combining Galadriel and Elrond’s genetics wasn’t enough, now they’re throwing Numenor in as well? Rude.”
I’m not saying that this discovery is why Sauron/Necromancer let himself get banished from Mirkwood back to Mordor. But it totally had everything to do with getting distance between them.