It's Sauron. The show is about him way back when before he was an eye in the sky and when he was forging the rings of power (thus the title). At least that's the impression I've gotten from the marketing.
But when it comes down to it, an early teaser doesn't need to tell you any more because (a) there will be lots more (tons more, so much you'll be sick of it) marketing as we get closer to the show in September and (b) when it launches everybody will be talking about it. Everybody. This teaser is just a reminder it's coming, nothing more.
Don't worry, you won't be left in the dark wondering whether you should watch this show or not. You won't be able to escape info about it. It's the most expensive show ever made. They'll be on every chat show. There will be ads and teasers and trailers everywhere. Every podcast and website will have breakdowns and rumours and summaries and previews. The news will talk about it!
I imagine we'll get to the point where we'll be avoiding previews and stuff due to spoilers. Luckily right now we're just at the early 'bookmark' stage of the marketing.