That's the trailer for Pokémon Nova Sun and Umbra Moon, a massive ROM hack of Pokemon Sun/Moon. As you can see, there's all sorts of changes here, amounting to a much more challenging experience. I'm sure Nintendo will squash this one, but at least you had a chance to check it out!
"From the creator of Rutile Ruby and Star Sapphire"
Welp, I'm out. I tried giving those a chance for improved difficulty and while they were harder, it was to annoying grinding levels. The game throws competitive team comps at you even early on, when you have no TMs to combat them. So you're being destroyed until you get a decent selection of TMs that actually help you, but by the time you get that far, you're mostly done with the game and are overleveled due to it being the only way to progress.
Why do fans even bother advertising this stuff? They KNOW Nintendo is going to take 'em down without mercy before long.