This is probably fake, with 95% certainty. Explanation:

The most obvious issue is the artwork. No offense to the artist, as this is at least pretty cool fanart, but the quality is rather poor compared to actual promotional material, most obvious in Luz’s face. I mean, look at her, just look into her eyes…

I do say whoever drew it did an okay job at making it look like Luz, but That’s Not My Baby

The owl beasts are also off model–Eda’s tail does not look like that, it’s more of a short fluffy tuft than a bird-like tail fan. Since The Owl House is script-oriented rather than board-oriented, it’s safe to assume there’s not just going to be artistic licence sprinkled around like that like you’d see in, say, a Cartoon Network show; even if it did, a whole different tail would be a bit extreme.

And that’s not even getting into the proportional issues with other characters, where thus far The Owl House has generally had very consistent proportions and relative scale between characters. Even if scale wasn’t an issue, King’s proportions in particular are very off.

On a more subtle note, the lighting in the water doesn’t seem quite right, almost more Steven Universe-like than Owl House-like, but then again the two shows share some of their background art crew; still, the water should be more pink/violet, not blue.

Art aside, while returning in early January isn’t completely unrealistic, that specific date would be a Friday, which is at least uncharacteristic. There is also a D23 expo logo; however, there is not a D23 expo coming up soon enough that there would likely already be leaks from it.

I’d go deeper and find the exact origin point of the image, but unfortunately I do not yet have anything deeper than the screenshot.

Anyway, I have concluded that this is fanart. Pretty cool fanart, and featuring a pretty neat prediction (I think we all want some monster Lily), but fanart nonetheless.